Author's Accepted Manuscript
Ultrasensitive detection of target analyteinduced aggregation of gold nanoparticles using laser-induced nanoparticle Rayleigh scattering Jia-Hui Lin, Wei-Lung Tseng
www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(14)00736-X http://dx.doi.org/10.1016/j.talanta.2014.08.055 TAL15066
To appear in:
Talanta
Received date: 18 June 2014 Revised date: 19 August 2014 Accepted date: 20 August 2014 Cite this article as: Jia-Hui Lin, Wei-Lung Tseng, Ultrasensitive detection of target analyte-induced aggregation of gold nanoparticles using laser-induced nanoparticle Rayleigh scattering, Talanta, http://dx.doi.org/10.1016/j.talanta.2014.08.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrasensitive detection of target analyte-induced aggregation of gold nanoparticles using laser-induced nanoparticle Rayleigh scattering.
Jia-Hui Lin1, and Wei-Lung Tseng*1,2, 3
1. Department of Chemistry, National Sun Yat-sen University, Taiwan 2. School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, Taiwan 3. National Sun Yat-sen University and Center for Nanoscience &Nanotechnology
Correspondence: Dr. Wei-Lung Tseng, Department of Chemistry, National Sun Yat-sen University, 70, Lien-hai Road, Kaohsiung, Taiwan 804. E-mail: [email protected] Fax: 011-886-7-3684046.
Abstract Detection of salt- and analyte-induced aggregation of gold nanoparticles (AuNPs) mostly relies on costly and bulky analytical instruments. To response this drawback, a portable, miniaturized, sensitive, and cost-effective detection technique is urgently required for rapid field detection and monitoring of target analyte via the use of 1
AuNP-based sensor. This study combined a miniaturized spectrometer with a 532-nm laser to develop a laser-induced Rayleigh scattering technique, allowing the sensitive and selective detection of Rayleigh scattering from the aggregated AuNPs. Three AuNP-based sensing systems, including salt-, thiol- and metal ion-induced aggregation of the AuNPs, were performed to examine the sensitivity of laser-induced Rayleigh scattering technique. Salt-, thiol-, and metal ion-promoted NP aggregation were exemplified by the use of aptamer-adsorbed, fluorosurfactant-stabilized, and gallic acid-capped AuNPs for probing K+, S-adenosylhomocysteine hydrolase-induced hydrolysis of S-adenosylhomocysteine, and Pb2+, in sequence. Compared to the reported methods for monitoring the aggregated AuNPs, the proposed system provided distinct advantages of sensitivity. Laser-induced Rayleigh scattering technique was improved to be convenient, cheap, and portable by replacing a diode laser and a miniaturized spectrometer with a laser pointer and a smart-phone. Using this smart-phone-based detection platform, we can determine whether or not the Pb2+ concentration exceed the maximum allowable level of Pb2+ in drinking water. Keywords: Laser-induced Rayleigh scattering; gold nanoparticles; smart phone; sensor
2
1. Introduction
Numerous nanomaterials, including magnetic nanoparticles (NPs), metal NPs, quantum dots and carbon-based nanomaterials, have served as signal transducers in a number of reported biosensors because they exhibit high surface area-to-volume ratio, strong signal intensities, and tunable surface chemistry [1-3]. Gold nanoparticles (AuNPs) provide several physical and chemical advantages, making them one of the most popular materials for the fabrication of chemical and biological sensors. First, a number of chemical approaches have been available for a one-pot synthesis of highly stable AuNPs [4]. Second, they possess surface Plasmon resonance (SPR), Rayleigh scattering, electric conductance, redox behavior, and enzyme-mimetic activity [4-8]. Third, they offer extremely large ensemble surface areas with tunable modification options [9]. Fourth, these properties of AuNPs can be readily tuned by varying their size, shape, and the surrounding chemical environment [4]. With these advantages, the binding event between the recognition element and the analyte can cause the change in physicochemical properties of transducer AuNPs, such as the intensity and wavelength of SPR [10], the magnitude of conductivity [11], and the behavior of redox reaction [6]. These changes in turn can produce a detectable response signal for
3
detecting a variety of analyte, such as heavy metal ions, anions, small molecules, proteins, and nucleic acids [4, 8, 10, 12, 13].
Because the optical properties of SPR of the AuNPs are strongly dependent on interparticle separation distances and particle size, the current gold standard for detecting the analyte-induced aggregation and growth of the AuNPs is to measure the wavelength and intensity of SPR, respectively. For example, the analyte-stimulated aggregation of the AuNPs drives interparticle surface plasmon coupling, causing a red-shift in the SPR peak [14]. Moreover, the H2O2- and dopamine-mediated enlargement of the AuNPs produced an increase in the intensity of SPR peak [15, 16]. However, most of these sensors exhibit moderate sensitivity with detection limits for target analyte in the micromolar range. The plasmon scattering intensity of a single 60-nm diameter AuNPs resembles the fluorescence intensity of 105 fluorescein molecules. The strong scattering light of the AuNPs arises from the collective oscillation of conducting electrons. Thus, to improve the sensitivity of AuNP-based colorimetric assays, numerous sensors are established based on the measurement of size-induced change in the maximum peak and intensity of a resonant Raleigh scattering [17]. For example, as compared to the dispersed AuNPs, the analyte-triggered aggregation of the AuNPs can scatter electromagnetic radiation strongly; this change can be monitored using dark-field light scattering [18], dynamic 4
light scattering [19], hyper-Rayleigh scattering [20], and resonance Rayleigh scattering techniques [21]. Dark-field light scattering technique can observe the scattering light of the AuNPs at the single-particle level, but such technique are limited to the detection of relatively large particles (> 40 nm) and the quantification of target analyte. Additionally, dynamic light scattering, hyper-Rayleigh scattering, and resonance Rayleigh scattering techniques are rather costly, time-consuming procedure, sophisticated, and non-portable.
Herein, we reported a highly sensitive light-scattering technique for sensing K+, S-adenosylhomocysteine hydrolase (SAHH) activity, and Pb2+ based on the fact that salt- thiol-, and metal ion-induced aggregation of the AuNPs greatly enhances light scattering, respectively. The measurement of light scattering is easy to perform using a miniaturized spectrometer as a scattering detector and a 532-nm laser as an excitation light. The integrated scattering spectrum was obtained by placing the detector at right angle to the excitation beam. The proposed method can provide more than 10-fold improvement in limit of detection (LOD) compared to colorimetric assay.
5
2. Experimental 2.1 Chemicals. S-adenosylhomocysteine (SAH), SAHH (rabbit erythrocytes; 50 000 units/L; MW 240 000), homocysteine (HCys), bovine serum albumin (BSA, from bovine serum), human serum albumin (HSA, from human serum), trypsin (from porcine pancreas), lysozyme (from human milk), thrombin (from bovine plasma), myoglobin (from equine heart), cytochrome c (from bovine heart), hemoglobin (from bovine blood), fluorosurfactant (FSN), dithiothreitol (DTT), trisodium citrate, HAuCl4, gallic acid, NaH2PO4, Na2HPO4, NaCl, NaClO4, MgCl2, FeCl3, NH4HCO3, Cd(ClO4)2, NaOH, Pb(NO3)2, and HgCl2 were obtained from Sigma-Aldrich (St. Louis, MO). FeCl2, CuCl2, SrCl2, LiCl, KCl, CoCl2, NiCl2, CaCl2, MnCl2, BaCl2, ZnCl2 were obtained from Acros Organics (Geel, Belgium). The molecular formula of FSN is F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH. Gold nanoparticles (15, 30, 50 nm) were purchased from Ted Pella Inc. (Redding, California). DNA sample (K+ aptamer, 5'-GGG TTA GGG TTA GGG TTA GGG-3') was synthesized from GenScript Corporation (Taipei, Taiwan). Isotonic phosphate-buffered saline (1× PBS; pH 7.4) was prepared by dissolving Na2HPO4·2H2O (22.05 g), NaH2PO4·H2O (2.07 g), and NaCl (4.5 g) in H2O (1.0 L). Water used in all of the experiments was doubly distilled 6
and purified by a Milli-Q system (Millipore, Milford, MA, USA).
2.2 Synthesis of 4 nm-sized AuNPs. The preparation of AuNPs was conducted by adding NaBH4 (0.1 M, 1.5 mL) to a solution (50 mL) containing 250 ȝM HAuCl4 and 250 ȝM trisodium citrate under vigorous stirring. The image of transmission electron microscopy (Tecnai 20 G2 S-Twin, Philips/FEI, Hillsboro, Oregon) demonstrated the size of the as-prepared AuNPs (Fig. S1, Supplementary information).
2.3 Apparatus A diode-pumped solid state continuous laser (Labguide Co.,Ltd) with a wavelength of 532 nm was used as an excitation light source when its output power was 5 mW. A miniaturized QE65000 Scientific-grade Spectrometer (Ocean Optics, Inc.) was used for collecting the scattering of the AuNPs. An Ocean Optics CUV-ALL-UV four-way cuvette holder (Ocean Optics, Inc.) equipped with fiber-optic couplings at each of four quartz f/2 collimating lenses was incorporated to a 1000 µm illumination fiber and a 1000 µm read fiber. An illumination fiber brings the excitation beam to a cuvette, while a read fiber transports the scattering signal back to Ocean Optics QE65000 spectrometer. The scattering spectrum was recorded using Ocean optics 7
SpectraSuite spectroscopy software with 100 ms integration time. To reduce the cost of instrumentation and toward more portable system, a battery-operated green laser point (532 nm) and a smart-phone (Samsung, Galaxy SIII) were used in place of a diode-pumped solid state continuous laser and a Ocean Optics QE65000 spectrometer, respectively.
2.4 Sensing of potassium ion. Citrate-capped AuNPs (15 nm; 1.5 nM, 10 mL) were modified with K+ aptamer (100 ȝM, 100 ȝL) for 2 h at ambient temperature. Metal ions (4−5000 ȝM) reacted with aptamer-modified AuNPs (0.75 nM) for 5 min at ambient temperature. The resulting solutions were incubated with 0.1× PBS solution for 5 min at ambient temperature. The scattering spectra were collected using the proposed detection system.
2.5 Sensing of SAHH Activity. Citrate-capped AuNPs (15 nm; 2.5 nM, 60 mL) were modified with FSN (10% w/v, 240 ȝL). FSN-stabilized AuNPs were stored at 4°C until further use. Proteins (0−100 units/L, 100 ȝL) reacted with SAH (100 ȝM, 100 ȝL) in 50 mM phosphate buffer (pH 7.2) at 37°C for 20 min. We incubated the resulting solutions (200 ȝL) with a solution (800 ȝL) containing FSN-AuNPs (0.1 nM) and phosphate buffer (100 mM, pH 5) for 8
10 min and recorded their scattering spectra.
2.6 Sensing of Lead Ion The pH of HAuCl4 was adjusted to 11.1 by adding 100 ȝL of 0.5 M NaOH. A solution of gallic acid was heated to 50°C. Subsequently, gallic acid (0.1 mL, 38.8 mM) was added slowly to HAuCl4 (10 mL, 1 mM) under vigorous stirring at ambient temperature for 6 h. The particle size of the formed AuNPs was 9 ± 1 nm. To estimate the concentration of gallic acid-capped AuNPs, we assumed that the reduction from gold(III) to gold atoms was 100% complete. According to this hypothesis, the concentration of gallic acid-capped AuNPs was calculated to be 40 nM. Metal ions (1−40 nM) were incubated with a solution containing gallic acid-capped AuNPs (0.63 nM), NaClO4 (10 mM) and formic acid buffer (pH 4.5, 20 mM) for 20 min at ambient temperature. The scattering spectra were collected using the proposed detection system.
9
3. Result and discussion RESULT AND DISCUSSION. 3.1 Sensing of K+ by salt-induced NP aggregation.
Because the scattering cross section of a single AuNPs is proportional to the sixth power of its diameter [22], we initially investigated the effect of different-sized AuNPs (4, 15, 30, and 50 nm) on their light-scattering intensity in deionized water and 0.5× PBS solution under irradiation with a 532-nm diode laser. The concentrations of different-sized AuNPs were all adjusted to 108 particles mL-1. Note that the AuNPs are dispersed in deionized water, whereas they aggregated to large-sized particles in 0.5× PBS. Salt-induced aggregation of the AuNPs is attributed to the fact that salt efficiently screens the surface charges of individual AuNP [23]. Fig. 1 shows a comparison of difference in light-scattering intensity between the dispersed and aggregated AuNPs. Expectably, the light-scattering intensity of the dispersed AuNPs tremendously increased with increasing their particle size, which is in agreement with Mie or Rayleigh scattering theory. Following the addition of 0.5× PBS to 4, 15, 30, and 50 nm of the dispersed AuNPs, we observed a 8-, 30-, 5-, and 1-fold increase in the light-scattering intensity at 532 nm, respectively. For larger particles (30 and 50 nm), the light-scattering intensity at 532 nm was very strong even
10
in the absence of salt, thus failing to measure the presence of the aggregated AuNPs. For smaller particles (4 nm), the aggregated AuNPs were too small to exhibit strong light-scattering intensity. We suggest that 15 nm AuNPs were more suitable for salt-induced aggregation of AuNP probes, owing to low background scattering from the dispersed AuNPs and an enormous increase in light-scattering intensity from the aggregated AuNPs.
(Figure 1)
Previous studies demonstrated that salt-induced aggregation of citrate-capped AuNPs was implemented for selective and sensitive detection of K+, thrombin, and adenosine
triphosphate
[24-26].
This
was
exemplified
with
the
use
of
aptamer-modified AuNPs for detecting K+ (Fig. 2A). Citrate-capped AuNPs interacting with aptamer are dispersed well under high-ionic-strength conditions. The presence of K+ induces the formation of K+−aptamer complexes, enabling aptamer to be removed from the NP surface [24, 27-34]. Because of the removal of aptamer from the NP surface, salt-induced aggregation of the AuNPs occurs and causes an enhancement in light-scattering intensity. Because the enhancement of the light-scattering intensity mainly arose from the degree of the NP aggregation, the effect of salt concentration on the stability of aptamer-modified AuNPs was explored
11
in the next study. Fig. S2 (Supplementary information) shows that aptamer-modified AuNPs remained dispersed when the concentration of PBS varied from 0 to 0.2× PBS. Thus, in 0.1× PBS, the scattering intensity of aptamer-modified AuNPs was measured before and after the addition of K+. As the concentration of K+ increased at fixed concentrations of citrate-capped AuNPs and aptamer, the scattering intensity of aptamer-capped AuNPs was progressively enhanced (Fig. 2B). The linear relationship (R2 = 0.9965) of the light-scattering intensity at 532 nm versus the K+ concentration was from 4 to 5000 ȝM (inset in Figure 2B). Because 0.1× PBS contains 17 mM Na+ ions, we suggest that this system is capable of selectively detecting 4 ȝM K+ in the presence of a 21000-fold excess of Na+ ions. This system enabled the detection of K+ with limit of detection (LOD) corresponding to 1 ȝM, which is much lower than the LOD values obtained from aptamer-based colorimetric, electrochemical, and fluorescent sensors probes [24, 27-34] (Table 1). Although dynamic light scattering, hyper-Rayleigh scattering, and resonance Rayleigh scattering techniques can be also utilized for observing salt-induced NP aggregation, the developed system consisting of miniaturized QE65000 spectrometer and diode laser can be portable and is interfaced to a personal computer via USB port. We next evaluated the specificity of the aptamer-based probe by monitoring the light-scattering intensity in the presence of other cation ions. Figure 2C shows that only K+ ions induced a remarkable change in 12
light-scattering intensity because other cations were unable to remove aptamer from the NP surface. The combination of laser-induced Rayleigh scattering technique and aptamer-based sensor therefore provided high sensitivity and selectivity for K+ ions.
(Figure 2)
3.2 Sensing of SAHH activity by homocysteine-mediated NP aggregation.
Numerous previous studies reported that aminothiol triggered the aggregation of nonionic FSN-stabilized AuNPs through interparticle H-bonds and electrostatic attraction between adsorbed aminothiol molecules.[35] The successful monitoring of salt-induced NP aggregation suggests that laser-induced Rayleigh scattering technique may be implemented to follow aminothiol-induced NP aggregation. In the presence of homocysteine, FSN-stabilized AuNPs with size of 15 nm provided relatively large difference in light-scattering intensity between the dispersed AuNPs and aggregated AuNPs as compared to 4, 30, and 50 nm-sized AuNPs (Figure S2, Supplementary information). Thus, we proposed that this system was utilized to detect the activity of SAHH. Figure 3A illustrates that SAHH catalyzes the hydrolysis of SAH to produce homocysteine and adenosine [30, 36]. The produced homocysteine, at low pH condition, triggers the aggregation of FSN-stabilized AuNPs, leading to strong 13
light-scattering under the excitation at 532 nm. As the concentration of SAHH increased at a fixed concentration of SAH, the light-scattering intensity at 532 nm of FSN-stabilized AuNPs was gradually enhanced (Figure 3B). By plotting the light-scattering intensity at 532 nm against the SAHH concentration, a linear range (R2 = 0.9962) for quantification of SAHH was observed from 15 to 50 units/ L-1. The LOD of SAHH was determined to be 10 units L-1 (approximately 0.6 nM). Our proposed system provided the greatest sensitivity toward SAHH activity among other reported methods, including AuNP-based colorimetric assay [36], UV-Vis absorption [37], Ellman′s reagent [36], and luciferase-based assay[38] (Table 2). Additionally, the combination of FSN-stabilized AuNPs and laser-induced Rayleigh scattering technique has great potential for sensing SAHH activity in red blood cells.[39] Other proteins were examined to determine the selectivity of our analytical system toward SAHH. Figure 3C shows that SAHH caused a dramatic change in the light-scattering intensity, whereas the remaining proteins exhibited a negligible effect under an identical condition. According the abovementioned results, we suggest that our analytical system is well suited to monitor thiol product-related enzyme reactions, such as glutathione reductase-catalyzed cleavage of glutathione disulfide [40], homocysteine thiolactonase-induced hydrolysis of homocysteine thiolactone [41], and acetylcholinesterase-triggered hydrolysis of acetylthiocholine [42]. 14
(Figure 3)
3.3 Sensing of Pb2+ by interparticle crosslinking aggregation.
Because the threat of heavy-metal pollution in environmental water is still an important issue for public health, laser-induced Rayleigh scattering technique was used to detect the conditions in which metal ions act as a bridge to link ligand-modified AuNPs together. Intensive researches revealed that AuNP-based colorimetric sensors were simple, selective, and sensitive to detect heavy metal ions in environmental waters [10, 12]. This was exemplified by the use of gallic acid-capped AuNPs for probing Pb2+ in an aqueous solution [43-45]. Figure 4A illustrates that Pb2+-stimulated aggregation of gallic acid-capped AuNPs results from multivalent coordination between Pb2+ and phenolic hydroxyl groups of gallic acid [43-45]. As shown in Figure 4B, the light-scattering intensity at 532 nm of gallic acid-capped AuNPs incrementally increased with increasing the concentration of Pb2+. The calibration curve for quantifying Pb2+ was obtained by plotting the light-scattering intensity at 532 nm versus the concentrations of Pb2+ over the range from 1 to 40 nM (inset in Figure 4B). The LOD of Pb2+ was estimated to be 0.3 nM, which is superior to the LOD values measured from AuNP-based colorimetric sensors [7, 43-45], 15
nanoparticle−DNAzyme-based
colorimetric
probes
[46-49],
AuNP-based
hyper-Rayleigh scattering sensor [50], and AuNP-based dynamic light scattering sensor [51] (Table 3). Additionally, the LOD of the proposed method is much lower than the maximum level (15 ȝg L-1 ~ 72 nM) of lead in drinking water permitted by the United State Environmental Protection Agency. Figure 4C depicts that only addition of Pb2+ to a solution of gallic acid-capped AuNPs caused a dramatic change in the light-scattering intensity at 532 nm, signifying that our analytical system exhibited excellent selectivity toward Pb2+ over other metal ions. We suggest that the combination of laser-induced Rayleigh scattering technique and AuNP-based metal ion sensor could be used to probe other metal ions by modified specific ligand onto the surface of the AuNPs.
(Figure 4)
3.4 Smart-phone-based heavy metal ions reader.
To further reduce the cost of instrumentation and toward more portable system, a laser pointer (< 20 mW) and a smart-phone were substituted for a diode laser and a miniaturized spectrometer, respectively (Fig. 5A). Fig. 5B shows a representative smart-phone-captured image of solutions of gallic acid-capped AuNPs in the absence 16
and presence of 72 nM Pb2+, i.e., the safety level recommended by U. S. EPA, in drinking water. The illumination intensity of the laser pointer was sufficient to pass through the sample and control cuvettes, forming two readable signals in two image frames. To determine whether the contamination of Pb2+ in drinking water, the acquired scattering images (164 × 5 pixels) of these cuvettes were converted to pixel intensity (0−256 counts) via Image J (Figure 5C). The intensity ratio of the sample to the control cuvette was calculated by the following equation:
Ratio = (I1/A1) / (I0/A0)
I1 and I0 are the integrated pixel intensities of the selected images in the sample and control cuvettes, respectively, while A1 and A0 are the area of the selected images in the sample and control cuvette. The intensity ratio of the sample to the control cuvette was computed to be 1.5, which corresponds to the maximum allowable level of Pb2+ in drinking water. In other words, if the intensity ratio of the sample to the control cuvette is higher than 1.5, samples of drinking water will contain above 72 nM Pb2+. Thus, the combination of a laser point, a smart-phone, and a AuNP-based probe was suited to monitor the maximum allowable levels of Pb2+ in drinking water defined by the U. S. EPA..
(Figure 5) 17
4. Conclusion
We have reported a miniaturized, sensitive, and cost-effective platform for monitoring the change in light scattering from three AuNP-based sensing systems, including salt-, thiol-, and metal ions-induced aggregation of the AuNPs. Salt-induced aggregation is exemplified by the use of aptamer-modified AuNPs for sensing K+ in a high-ionic-strength solution. Laser-induced Rayleigh scattering technique coupled to aptamer-based sensor provided high sensitivity (LOD = 1 ȝM) and selectivity (21000-fold) for K+ ions. Thiol-triggered aggregation of the AuNPs is exemplified by the use of FSN-stabilized AuNPs for probing homocysteine, which is generated from SAHH-induced hydrolysis of SAH. The proposed detection system combined with FSN-stabilized AuNPs was highly responsive and selective for the SAHH/SAH system. Metal ion-promoted assembly of the AuNPs is exemplified by the detection of Pb2+ with gallic acid-capped AuNPs. By combining laser-induced Rayleigh scattering technique with gallic acid-capped AuNPs, the LOD of Pb2+ was down to 0.3 nM. Also, a sensitive, portable cost-effective smart-phone-based Pb2+ sensor platform was developed by replacing a diode laser and a miniaturized spectrometer with a laser pointer and a smart-phone. Previous studies reported that Cr3+ and Hg2+
18
triggered
the
aggregation
of
5-thiol-(2-nitrobenzoic
acid)-
and
3-mercaptopropionate acid-capped AuNPs [8, 52]. In the opinion of the authors, the smart-phone-based detection system combined with 5-thiol-(2-nitrobenzoic acid)- and 3-mercaptopropionate acid-capped AuNPs could be implemented for sensing Cr3+ and Hg2+ in an aqueous solution, respectively.
Acknowledgment
We would like to thank National Science Council (NSC 100-2628-M-110-001-MY 4) for the financial support of this study.
References
[1] J.S. Beveridge, J.R. Stephens, M.E. Williams, Annu. Rev. Anal. Chem. 4 (2011) 251. [2] D. Jariwala, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, Chem. Soc. Rev. 42 (2013) 2824. [3] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Nat. Mater. 4 (2005) 435. [4] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. 19
[5] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2002) 668. [6] W.J. Luo, C.F. Zhu, S. Su, D. Li, Y. He, Q. Huang, C.H. Fan, ACS Nano 4 (2010) 7451. [7] F. Chai, C.A. Wang, T.T. Wang, L. Li, Z.M. Su, ACS Appl. Mater. Inter. 2 (2010) 1466. [8] C.J. Yu, W.L. Tseng, Langmuir 24 (2008) 12717. [9] E. Katz, I. Willner, Angew. Chem. Int. Ed. 43 (2004) 6042. [10] G. Aragay, J. Pons, A. Merkoci, Chem. Rev. 111 (2011) 3433. [11] S.J. Park, T.A. Taton, C.A. Mirkin, Science 295 (2002) 1503. [12] D. Liu, Z. Wang, X. Jiang, Nanoscale 3 (2011) 1421. [13] E. Hutter, D. Maysinger, Trends Pharmacol. Sci. 34 (2013) 497. [14] D. Vilela, M.C. Gonzalez, A. Escarpa, Anal. Chim. Acta 751 (2012) 24. [15] M. Zayats, R. Baron, I. Popov, I. Willner, Nano Lett. 5 (2005) 21. [16] R. Baron, M. Zayats, I. Willner, Anal. Chem. 77 (2005) 1566. [17] Y. Li, C. Jing, L. Zhang, Y.T. Long, Chem. Soc. Rev. 41 (2012) 632. [18] T. Bu, T. Zako, M. Fujita, M. Maeda, Chem. Commun. 49 (2013) 7531. [19] X. Liu, Q. Dai, L. Austin, J. Coutts, G. Knowles, J. Zou, H. Chen, Q. Huo, J. Am. Chem. Soc. 130 (2008) 2780. 20
[20] G.K. Darbha, A.K. Singh, U.S. Rai, E. Yu, H. Yu, P. Chandra Ray, J. Am. Chem. Soc. 130 (2008) 8038. [21] B.A. Du, Z.P. Li, C.H. Liu, Angew. Chem. Int. Ed. 45 (2006) 8022. [22]M.A. van Dijk, A.L. Tchebotareva, M. Orrit, M. Lippitz, S. Berciaud, D. Lasne, L. Cognet, B. Lounis, Phys. Chem. Chem. Phys. 8 (2006) 3486. [23] W. Zhao, M.A. Brook, Y.F. Li, Chembiochem 9 (2008) 2363. [24] L. Wang, X. Liu, X. Hu, S. Song, C. Fan, Chem. Commun. (2006) 3780. [25] H. Wei, B. Li, J. Li, E. Wang, S. Dong, Chem. Commun. (2007) 3735. [26] J. Wang, L. Wang, X. Liu, Z. Liang, S. Song, W. Li, G. Li, C. Fan, Adv. Mater. 19 (2007) 3943. [27] X. Zhu, J. Zhao, Y. Wu, Z. Shen, G. Li, Anal. Chem. 83 (2011) 4085. [28] Z.S. Wu, C.R. Chen, G.L. Shen, R.Q. Yu, Biomaterials 29 (2008) 2689. [29] C. Shi, H. Gu, C. Ma, Anal. Biochem. 400 (2010) 99. [30] D.M. Kong, J.H. Guo, W. Yang, Y.E. Ma, H.X. Shen, Biosens. Bioelectron. 25 (2009) 88. [31] S. Nagatoishi, T. Nojima, B. Juskowiak, S. Takenaka, Angew. Chem. Int. Ed. 44 (2005) 5067. [32] S. Nagatoishi, T. Nojima, E. Galezowska, A. Gluszynska, B. Juskowiak, S. Takenaka, Anal. Chim. Acta 581 (2007) 125. 21
[33] F. He, Y. Tang, S. Wang, Y. Li, D. Zhu, J. Am. Chem. Soc. 127 (2005) 12343. [34] A.E. Radi, C.K. O'Sullivan, Chem. Commun. (2006) 3432. [35] Lim, II, W. Ip, E. Crew, P.N. Njoki, D. Mott, C.J. Zhong, Y. Pan, S. Zhou, Langmuir 23 (2007) 826. [36] J.H. Lin, C.W. Chang, Z.H. Wu, W.L. Tseng, Anal. Chem. 82 (2010) 8775. [37] J.L. Palmer, R.H. Abeles, J. Biol. Chem. 254 (1979) 1217. [38] E.S. Burgos, S.A. Gulab, M.B. Cassera, V.L. Schramm, Anal. Chem. 84 (2012) 3593. [39] F. Carlucci, A. Tabucchi, A. Aiuti, F. Rosi, F. Floccari, R. Pagani, E. Marinello, Clin. Chem. 49 (2003) 1830. [40] P. Monostori, G. Wittmann, E. Karg, S. Turi, J. Chromatogr. B 877 (2009) 3331. [41] J. Perla-Kajan, H. Jakubowski, Amino Acids 43 (2012) 1405. [42] Y. Miao, N. He, J.J. Zhu, Chem. Rev. 110 (2010) 5216. [43] K.W. Huang, C.J. Yu, W.L. Tseng, Biosens. Bioelectron. 25 (2010) 984. [44] K. Yoosaf, B.I. Ipe, C.H. Suresh, K.G. Thomas, J. Phys. Chem. C 111 (2007) 12839. [45] N. Ding, Q. Cao, H. Zhao, Y. Yang, L. Zeng, Y. He, K. Xiang, G. Wang, Sensors 10 (2010) 11144. [46] J. Liu, Y. Lu, J. Am. Chem. Soc. 126 (2004) 12298. 22
[47] H. Wei, B. Li, J. Li, S. Dong, E. Wang, Nanotechnology 19 (2008) 095501. [48] D. Mazumdar, J. Liu, G. Lu, J. Zhou, Y. Lu, Chem. Commun. 46 (2010) 1416. [49] S.Y. Lin, S.H. Wu, C.H. Chen, Angew. Chem. Int. Ed. 45 (2006) 4948. [50] Y. Kim, R.C. Johnson, J.T. Hupp, Nano Lett. 1 (2001) 165. [51] L. Beqa, A.K. Singh, S.A. Khan, D. Senapati, S.R. Arumugam, P.C. Ray, ACS Appl. Mater. Interfaces 3 (2011) 668. [52] Y.J. Lai, W.L. Tseng, Analyst 136 (2011) 2712.
23
Figure 1. Scattering spectra of (A) 3.5 nm-, (B) 15 nm-, (C) 30 nm-, (D) 50 nm-sized AuNPs in the (a) absence and (b) presence of 0.5× PBS. The concentration of the AuNPs was fixed at 108 particles mL-1.
24
Figure 2. (A) Illustration of the mechanism of aptamer-modified AuNPs for sensing K+ via salt-induced NP aggregation. (B) Scattering spectra obtained from the addition of aptamer-modified AuNPs and 4−5000 ȝM K+ to 0.1× PBS (pH 7.4). The arrows indicate the signal changes with increase in K+ concentration (4 , 10, 40, 1000, 5000 ȝM). (C) Scattering intensity at 520 nm obtained from the addition of a mixture of aptamer-modified AuNPs and analyte to 0.1× PBS (pH 7.4). Analyte: (a) 10 ȝM K+, (b) 5 mM Li+, (c) 5 mM Na+, (d) 5 mM Mg2+, (e) 5 mM Ca2+, and (f) 5 mM NH4+. Metal ions were incubated with aptamer-modified AuNPs (0.75 nM) for 5 min at ambient temperature. The resulting solutions were mixed with 0.1× PBS solution for 5 min at ambient temperature.
25
Figure 3. (A) Illustration of the mechanism of FSN-stabilized AuNPs for sensing SAHH activity via thiol-induced NP aggregation. (B) Scattering spectra of FSN-stabilized AuNPs obtained after the addition of a mixture of 50 ȝM SAH and 10−50 units/L SAHH. The arrows indicate the signal changes with increase in SAHH concentration (10, 15, 20, 25, 35, 40, and 50 units/L). (C) Scattering intensity at 520 nm obtained from the addition of FSN-stabilized AuNPs to a mixture of 1.5 nM protein and 50 ȝM SAH. Protein: (a) SAHH, (b) trypsin, (c) thrombin, (d) lysozyme, (e) myoglobin, (f) cytochrome c, (g) hemoglobin, (h) Human serum albumin, and (i) bovine serum albumin. (B, C) Proteins reacted with SAH in 50 mM phosphate buffer (pH 7.2) at 37°C for 20 min. The resulting solutions were incubated with a solution containing FSN-AuNPs (0.1 nM) and phosphate buffer (100 mM, pH 5) for 10 min at ambient temperature.
26
Figure 4. (A) Illustration of the mechanism of gallic acid-capped AuNPs for sensing Pb2+ via the coordination between gallic acid and Pb2+. (B) Scattering spectra of gallic acid-capped AuNPs in the presence of increasing Pb2+ concentration. The arrows indicate the signal changes with increase in Pb2+ concentration (1, 6, 10, 20, and 40 nM). (C) Scattering intensity at 520 nm obtained from the addition of metal ion to a solution of gallic acid-capped AuNPs. Metal ion: (a) Pb2+, (b) Li+, (c) Na+, (d) K+, (e) Mg2+, (f) Ca2+, (g) Sr2+, (h) Ba2+, (i) Fe2+, (j) Fe3+, (k) Co2+, (l) Ni2+, (m) Cu2+, (n) Zn2+, (o) Mn2+, (p) Hg2+, (q) Cd2+. The concentration of Pb2+ is 10 nM, while the concentration of other metal ion is 1 ȝM. (B, C) Metal ions were incubated with a solution containing gallic acid-capped AuNPs (0.63 nM), NaClO4 (10 mM) and formic acid buffer (pH 4.5, 20 mM) for 20 min at ambient temperature.
27
Figure 5. (A) Schematic illustration of the portable device for the determination of Pb2+ in drinking water. (B) Photo images captured on the smart-phone under laser illumination. The left cuvette contained a mixture of gallic acid-capped AuNPs (0.63 nM), NaClO4 (10 mM), formic acid buffer (pH 4.5, 20 mM), and drinking water, while the right cuvette contained the above mentioned mixtures in the presence of 72 nM Pb2+. (C) Flow of image-processing steps to compute normalized pixel intensity. Conversion of selected image to integrated pixel intensity by ImageJ. The integrated pixel intensity was normalized by dividing it by the sum of pixel areas.
28
Table 1. Comparison of other AuNP-based sensors for the determination of K+ Sensing systema
detection
MDCb
LODc
Reference
aptamer-modified AuNPs
Colorimetry
1 mM
N.A.
24
Nicking endonuclease-assisted
Colorimetry
0.1 mM N.A.
27
G-quadruplex DNA-based
Electrochemical
N.A.
28
electric switch
detection
aptamer/pyrene-labeled
Fluorescence
0.6 mM 0.4 mM
29
Fluorescence
0.1 mM N.A.
30
Fluorescence
0.5 mM N.A.
31
Fluorescence
2 mM
N.A.
32
aptamer-based DNA/CCPs
Fluorescence
2 mM
N.A.
33
G-quadruplex DNA-based
Electrochemical
0.1 mM N.A.
34
electric switch
detection
aptamer-modified AuNPs
Light scattering
4 ȝM
1 ȝM
This study
AuNPs sensor N.A.
molecular beacon G-quadruplex/crystal violet complexes Pyrene-labeled G-quadruplex DNA sensor G-quadruplex DNA fluorescent sensor
a
CCPs, cationic conjugated polymers; b LDC, minimum detectable concentration.
c
N. A., not available
29
Table 2. Comparison of other methods for the determination of SAHH Sensing system
Detection
MDCa
LODb
Reference
FSN-stabilized AuNPs
colorimetry
9 nM
6 nM
36
Ellman′s reagent
Absorption
2 nM.
N.A.
36
Luciferase-based assay
fluorescence
2 nM
N.A.
39
FSN-stabilized AuNPs
light scattering
0.9 nM
0.6 nM
This study
a
MDC, minimum detectable concentration. b N. A., not available
30
Table 3. compared of other methods for the determination of Pb2+. Sensing system
Detection
MDCb
LODb
Glutathione-capped AuNPs
Colorimetry
0.1 ȝM
100 nM 7
Gallic acid-capped AuNPs
Colorimetry
10 nM
N.A.
43
Gallic acid-capped AuNPs
Colorimetry
15 ȝM
N.A.
44
Gallic acid-capped AuNPs
Colorimetry
50 nM
25 nM.
45
DNAzyme, DNA-capped
Colorimetry
0.4 ȝM
N.A.
46
Colorimetry
N.A.
500 nM 47
Colorimetry
N.A.
0.5 ȝM
48
Crown ether-modified AuNPs
Colorimetry
0.25 ȝM
N.A.
49
11-mercaptoundecanoic
HRS
N.A.
25 ȝM
50
GSH-AuNPs
DLS
50 ppt
N.A.
51
Gallic acid-capped AuNPs
Light scattering
1 nM
0.3 nM
This study
Reference
AuNPs DNAzyme, citrate-capped AuNPs DNAzyme-AuNPs dipstick test
acid-modified AuNPs
a
HRS, Hyper-Reyleigh scattering; DLS, dynamic light scattering. b N.A., not
avaliable
31
z
Laser-induced Rayleigh scattering is sensitive to detect aggregated nanoparticles.
z z
The proposed system coupled to aptamer-capped AuNPs was sensitive for K+. The proposed system coupled to FSN-stabilized AuNPs was sensitive for
z
homocysteine. The proposed system coupled to gallic acid-capped AuNPs was sensitive for Pb2+.
z
The proposed system is convenient, cheap, and portable.
32
*Graphical Abstract (for review)
A sensitive, portable cost-effective smart-phone-based Pb2+ sensor platform was developed by a laser pointer, a smart-phone, and gallic acid-capped gold nanoparticles.
Ultrasensitive detection of target analyteinduced aggregation of gold nanoparticles using laser-induced nanoparticle Rayleigh scattering Jia-Hui Lin, Wei-Lung Tseng
www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(14)00736-X http://dx.doi.org/10.1016/j.talanta.2014.08.055 TAL15066
To appear in:
Talanta
Received date: 18 June 2014 Revised date: 19 August 2014 Accepted date: 20 August 2014 Cite this article as: Jia-Hui Lin, Wei-Lung Tseng, Ultrasensitive detection of target analyte-induced aggregation of gold nanoparticles using laser-induced nanoparticle Rayleigh scattering, Talanta, http://dx.doi.org/10.1016/j.talanta.2014.08.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrasensitive detection of target analyte-induced aggregation of gold nanoparticles using laser-induced nanoparticle Rayleigh scattering.
Jia-Hui Lin1, and Wei-Lung Tseng*1,2, 3
1. Department of Chemistry, National Sun Yat-sen University, Taiwan 2. School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, Taiwan 3. National Sun Yat-sen University and Center for Nanoscience &Nanotechnology
Correspondence: Dr. Wei-Lung Tseng, Department of Chemistry, National Sun Yat-sen University, 70, Lien-hai Road, Kaohsiung, Taiwan 804. E-mail: [email protected] Fax: 011-886-7-3684046.
Abstract Detection of salt- and analyte-induced aggregation of gold nanoparticles (AuNPs) mostly relies on costly and bulky analytical instruments. To response this drawback, a portable, miniaturized, sensitive, and cost-effective detection technique is urgently required for rapid field detection and monitoring of target analyte via the use of 1
AuNP-based sensor. This study combined a miniaturized spectrometer with a 532-nm laser to develop a laser-induced Rayleigh scattering technique, allowing the sensitive and selective detection of Rayleigh scattering from the aggregated AuNPs. Three AuNP-based sensing systems, including salt-, thiol- and metal ion-induced aggregation of the AuNPs, were performed to examine the sensitivity of laser-induced Rayleigh scattering technique. Salt-, thiol-, and metal ion-promoted NP aggregation were exemplified by the use of aptamer-adsorbed, fluorosurfactant-stabilized, and gallic acid-capped AuNPs for probing K+, S-adenosylhomocysteine hydrolase-induced hydrolysis of S-adenosylhomocysteine, and Pb2+, in sequence. Compared to the reported methods for monitoring the aggregated AuNPs, the proposed system provided distinct advantages of sensitivity. Laser-induced Rayleigh scattering technique was improved to be convenient, cheap, and portable by replacing a diode laser and a miniaturized spectrometer with a laser pointer and a smart-phone. Using this smart-phone-based detection platform, we can determine whether or not the Pb2+ concentration exceed the maximum allowable level of Pb2+ in drinking water. Keywords: Laser-induced Rayleigh scattering; gold nanoparticles; smart phone; sensor
2
1. Introduction
Numerous nanomaterials, including magnetic nanoparticles (NPs), metal NPs, quantum dots and carbon-based nanomaterials, have served as signal transducers in a number of reported biosensors because they exhibit high surface area-to-volume ratio, strong signal intensities, and tunable surface chemistry [1-3]. Gold nanoparticles (AuNPs) provide several physical and chemical advantages, making them one of the most popular materials for the fabrication of chemical and biological sensors. First, a number of chemical approaches have been available for a one-pot synthesis of highly stable AuNPs [4]. Second, they possess surface Plasmon resonance (SPR), Rayleigh scattering, electric conductance, redox behavior, and enzyme-mimetic activity [4-8]. Third, they offer extremely large ensemble surface areas with tunable modification options [9]. Fourth, these properties of AuNPs can be readily tuned by varying their size, shape, and the surrounding chemical environment [4]. With these advantages, the binding event between the recognition element and the analyte can cause the change in physicochemical properties of transducer AuNPs, such as the intensity and wavelength of SPR [10], the magnitude of conductivity [11], and the behavior of redox reaction [6]. These changes in turn can produce a detectable response signal for
3
detecting a variety of analyte, such as heavy metal ions, anions, small molecules, proteins, and nucleic acids [4, 8, 10, 12, 13].
Because the optical properties of SPR of the AuNPs are strongly dependent on interparticle separation distances and particle size, the current gold standard for detecting the analyte-induced aggregation and growth of the AuNPs is to measure the wavelength and intensity of SPR, respectively. For example, the analyte-stimulated aggregation of the AuNPs drives interparticle surface plasmon coupling, causing a red-shift in the SPR peak [14]. Moreover, the H2O2- and dopamine-mediated enlargement of the AuNPs produced an increase in the intensity of SPR peak [15, 16]. However, most of these sensors exhibit moderate sensitivity with detection limits for target analyte in the micromolar range. The plasmon scattering intensity of a single 60-nm diameter AuNPs resembles the fluorescence intensity of 105 fluorescein molecules. The strong scattering light of the AuNPs arises from the collective oscillation of conducting electrons. Thus, to improve the sensitivity of AuNP-based colorimetric assays, numerous sensors are established based on the measurement of size-induced change in the maximum peak and intensity of a resonant Raleigh scattering [17]. For example, as compared to the dispersed AuNPs, the analyte-triggered aggregation of the AuNPs can scatter electromagnetic radiation strongly; this change can be monitored using dark-field light scattering [18], dynamic 4
light scattering [19], hyper-Rayleigh scattering [20], and resonance Rayleigh scattering techniques [21]. Dark-field light scattering technique can observe the scattering light of the AuNPs at the single-particle level, but such technique are limited to the detection of relatively large particles (> 40 nm) and the quantification of target analyte. Additionally, dynamic light scattering, hyper-Rayleigh scattering, and resonance Rayleigh scattering techniques are rather costly, time-consuming procedure, sophisticated, and non-portable.
Herein, we reported a highly sensitive light-scattering technique for sensing K+, S-adenosylhomocysteine hydrolase (SAHH) activity, and Pb2+ based on the fact that salt- thiol-, and metal ion-induced aggregation of the AuNPs greatly enhances light scattering, respectively. The measurement of light scattering is easy to perform using a miniaturized spectrometer as a scattering detector and a 532-nm laser as an excitation light. The integrated scattering spectrum was obtained by placing the detector at right angle to the excitation beam. The proposed method can provide more than 10-fold improvement in limit of detection (LOD) compared to colorimetric assay.
5
2. Experimental 2.1 Chemicals. S-adenosylhomocysteine (SAH), SAHH (rabbit erythrocytes; 50 000 units/L; MW 240 000), homocysteine (HCys), bovine serum albumin (BSA, from bovine serum), human serum albumin (HSA, from human serum), trypsin (from porcine pancreas), lysozyme (from human milk), thrombin (from bovine plasma), myoglobin (from equine heart), cytochrome c (from bovine heart), hemoglobin (from bovine blood), fluorosurfactant (FSN), dithiothreitol (DTT), trisodium citrate, HAuCl4, gallic acid, NaH2PO4, Na2HPO4, NaCl, NaClO4, MgCl2, FeCl3, NH4HCO3, Cd(ClO4)2, NaOH, Pb(NO3)2, and HgCl2 were obtained from Sigma-Aldrich (St. Louis, MO). FeCl2, CuCl2, SrCl2, LiCl, KCl, CoCl2, NiCl2, CaCl2, MnCl2, BaCl2, ZnCl2 were obtained from Acros Organics (Geel, Belgium). The molecular formula of FSN is F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH. Gold nanoparticles (15, 30, 50 nm) were purchased from Ted Pella Inc. (Redding, California). DNA sample (K+ aptamer, 5'-GGG TTA GGG TTA GGG TTA GGG-3') was synthesized from GenScript Corporation (Taipei, Taiwan). Isotonic phosphate-buffered saline (1× PBS; pH 7.4) was prepared by dissolving Na2HPO4·2H2O (22.05 g), NaH2PO4·H2O (2.07 g), and NaCl (4.5 g) in H2O (1.0 L). Water used in all of the experiments was doubly distilled 6
and purified by a Milli-Q system (Millipore, Milford, MA, USA).
2.2 Synthesis of 4 nm-sized AuNPs. The preparation of AuNPs was conducted by adding NaBH4 (0.1 M, 1.5 mL) to a solution (50 mL) containing 250 ȝM HAuCl4 and 250 ȝM trisodium citrate under vigorous stirring. The image of transmission electron microscopy (Tecnai 20 G2 S-Twin, Philips/FEI, Hillsboro, Oregon) demonstrated the size of the as-prepared AuNPs (Fig. S1, Supplementary information).
2.3 Apparatus A diode-pumped solid state continuous laser (Labguide Co.,Ltd) with a wavelength of 532 nm was used as an excitation light source when its output power was 5 mW. A miniaturized QE65000 Scientific-grade Spectrometer (Ocean Optics, Inc.) was used for collecting the scattering of the AuNPs. An Ocean Optics CUV-ALL-UV four-way cuvette holder (Ocean Optics, Inc.) equipped with fiber-optic couplings at each of four quartz f/2 collimating lenses was incorporated to a 1000 µm illumination fiber and a 1000 µm read fiber. An illumination fiber brings the excitation beam to a cuvette, while a read fiber transports the scattering signal back to Ocean Optics QE65000 spectrometer. The scattering spectrum was recorded using Ocean optics 7
SpectraSuite spectroscopy software with 100 ms integration time. To reduce the cost of instrumentation and toward more portable system, a battery-operated green laser point (532 nm) and a smart-phone (Samsung, Galaxy SIII) were used in place of a diode-pumped solid state continuous laser and a Ocean Optics QE65000 spectrometer, respectively.
2.4 Sensing of potassium ion. Citrate-capped AuNPs (15 nm; 1.5 nM, 10 mL) were modified with K+ aptamer (100 ȝM, 100 ȝL) for 2 h at ambient temperature. Metal ions (4−5000 ȝM) reacted with aptamer-modified AuNPs (0.75 nM) for 5 min at ambient temperature. The resulting solutions were incubated with 0.1× PBS solution for 5 min at ambient temperature. The scattering spectra were collected using the proposed detection system.
2.5 Sensing of SAHH Activity. Citrate-capped AuNPs (15 nm; 2.5 nM, 60 mL) were modified with FSN (10% w/v, 240 ȝL). FSN-stabilized AuNPs were stored at 4°C until further use. Proteins (0−100 units/L, 100 ȝL) reacted with SAH (100 ȝM, 100 ȝL) in 50 mM phosphate buffer (pH 7.2) at 37°C for 20 min. We incubated the resulting solutions (200 ȝL) with a solution (800 ȝL) containing FSN-AuNPs (0.1 nM) and phosphate buffer (100 mM, pH 5) for 8
10 min and recorded their scattering spectra.
2.6 Sensing of Lead Ion The pH of HAuCl4 was adjusted to 11.1 by adding 100 ȝL of 0.5 M NaOH. A solution of gallic acid was heated to 50°C. Subsequently, gallic acid (0.1 mL, 38.8 mM) was added slowly to HAuCl4 (10 mL, 1 mM) under vigorous stirring at ambient temperature for 6 h. The particle size of the formed AuNPs was 9 ± 1 nm. To estimate the concentration of gallic acid-capped AuNPs, we assumed that the reduction from gold(III) to gold atoms was 100% complete. According to this hypothesis, the concentration of gallic acid-capped AuNPs was calculated to be 40 nM. Metal ions (1−40 nM) were incubated with a solution containing gallic acid-capped AuNPs (0.63 nM), NaClO4 (10 mM) and formic acid buffer (pH 4.5, 20 mM) for 20 min at ambient temperature. The scattering spectra were collected using the proposed detection system.
9
3. Result and discussion RESULT AND DISCUSSION. 3.1 Sensing of K+ by salt-induced NP aggregation.
Because the scattering cross section of a single AuNPs is proportional to the sixth power of its diameter [22], we initially investigated the effect of different-sized AuNPs (4, 15, 30, and 50 nm) on their light-scattering intensity in deionized water and 0.5× PBS solution under irradiation with a 532-nm diode laser. The concentrations of different-sized AuNPs were all adjusted to 108 particles mL-1. Note that the AuNPs are dispersed in deionized water, whereas they aggregated to large-sized particles in 0.5× PBS. Salt-induced aggregation of the AuNPs is attributed to the fact that salt efficiently screens the surface charges of individual AuNP [23]. Fig. 1 shows a comparison of difference in light-scattering intensity between the dispersed and aggregated AuNPs. Expectably, the light-scattering intensity of the dispersed AuNPs tremendously increased with increasing their particle size, which is in agreement with Mie or Rayleigh scattering theory. Following the addition of 0.5× PBS to 4, 15, 30, and 50 nm of the dispersed AuNPs, we observed a 8-, 30-, 5-, and 1-fold increase in the light-scattering intensity at 532 nm, respectively. For larger particles (30 and 50 nm), the light-scattering intensity at 532 nm was very strong even
10
in the absence of salt, thus failing to measure the presence of the aggregated AuNPs. For smaller particles (4 nm), the aggregated AuNPs were too small to exhibit strong light-scattering intensity. We suggest that 15 nm AuNPs were more suitable for salt-induced aggregation of AuNP probes, owing to low background scattering from the dispersed AuNPs and an enormous increase in light-scattering intensity from the aggregated AuNPs.
(Figure 1)
Previous studies demonstrated that salt-induced aggregation of citrate-capped AuNPs was implemented for selective and sensitive detection of K+, thrombin, and adenosine
triphosphate
[24-26].
This
was
exemplified
with
the
use
of
aptamer-modified AuNPs for detecting K+ (Fig. 2A). Citrate-capped AuNPs interacting with aptamer are dispersed well under high-ionic-strength conditions. The presence of K+ induces the formation of K+−aptamer complexes, enabling aptamer to be removed from the NP surface [24, 27-34]. Because of the removal of aptamer from the NP surface, salt-induced aggregation of the AuNPs occurs and causes an enhancement in light-scattering intensity. Because the enhancement of the light-scattering intensity mainly arose from the degree of the NP aggregation, the effect of salt concentration on the stability of aptamer-modified AuNPs was explored
11
in the next study. Fig. S2 (Supplementary information) shows that aptamer-modified AuNPs remained dispersed when the concentration of PBS varied from 0 to 0.2× PBS. Thus, in 0.1× PBS, the scattering intensity of aptamer-modified AuNPs was measured before and after the addition of K+. As the concentration of K+ increased at fixed concentrations of citrate-capped AuNPs and aptamer, the scattering intensity of aptamer-capped AuNPs was progressively enhanced (Fig. 2B). The linear relationship (R2 = 0.9965) of the light-scattering intensity at 532 nm versus the K+ concentration was from 4 to 5000 ȝM (inset in Figure 2B). Because 0.1× PBS contains 17 mM Na+ ions, we suggest that this system is capable of selectively detecting 4 ȝM K+ in the presence of a 21000-fold excess of Na+ ions. This system enabled the detection of K+ with limit of detection (LOD) corresponding to 1 ȝM, which is much lower than the LOD values obtained from aptamer-based colorimetric, electrochemical, and fluorescent sensors probes [24, 27-34] (Table 1). Although dynamic light scattering, hyper-Rayleigh scattering, and resonance Rayleigh scattering techniques can be also utilized for observing salt-induced NP aggregation, the developed system consisting of miniaturized QE65000 spectrometer and diode laser can be portable and is interfaced to a personal computer via USB port. We next evaluated the specificity of the aptamer-based probe by monitoring the light-scattering intensity in the presence of other cation ions. Figure 2C shows that only K+ ions induced a remarkable change in 12
light-scattering intensity because other cations were unable to remove aptamer from the NP surface. The combination of laser-induced Rayleigh scattering technique and aptamer-based sensor therefore provided high sensitivity and selectivity for K+ ions.
(Figure 2)
3.2 Sensing of SAHH activity by homocysteine-mediated NP aggregation.
Numerous previous studies reported that aminothiol triggered the aggregation of nonionic FSN-stabilized AuNPs through interparticle H-bonds and electrostatic attraction between adsorbed aminothiol molecules.[35] The successful monitoring of salt-induced NP aggregation suggests that laser-induced Rayleigh scattering technique may be implemented to follow aminothiol-induced NP aggregation. In the presence of homocysteine, FSN-stabilized AuNPs with size of 15 nm provided relatively large difference in light-scattering intensity between the dispersed AuNPs and aggregated AuNPs as compared to 4, 30, and 50 nm-sized AuNPs (Figure S2, Supplementary information). Thus, we proposed that this system was utilized to detect the activity of SAHH. Figure 3A illustrates that SAHH catalyzes the hydrolysis of SAH to produce homocysteine and adenosine [30, 36]. The produced homocysteine, at low pH condition, triggers the aggregation of FSN-stabilized AuNPs, leading to strong 13
light-scattering under the excitation at 532 nm. As the concentration of SAHH increased at a fixed concentration of SAH, the light-scattering intensity at 532 nm of FSN-stabilized AuNPs was gradually enhanced (Figure 3B). By plotting the light-scattering intensity at 532 nm against the SAHH concentration, a linear range (R2 = 0.9962) for quantification of SAHH was observed from 15 to 50 units/ L-1. The LOD of SAHH was determined to be 10 units L-1 (approximately 0.6 nM). Our proposed system provided the greatest sensitivity toward SAHH activity among other reported methods, including AuNP-based colorimetric assay [36], UV-Vis absorption [37], Ellman′s reagent [36], and luciferase-based assay[38] (Table 2). Additionally, the combination of FSN-stabilized AuNPs and laser-induced Rayleigh scattering technique has great potential for sensing SAHH activity in red blood cells.[39] Other proteins were examined to determine the selectivity of our analytical system toward SAHH. Figure 3C shows that SAHH caused a dramatic change in the light-scattering intensity, whereas the remaining proteins exhibited a negligible effect under an identical condition. According the abovementioned results, we suggest that our analytical system is well suited to monitor thiol product-related enzyme reactions, such as glutathione reductase-catalyzed cleavage of glutathione disulfide [40], homocysteine thiolactonase-induced hydrolysis of homocysteine thiolactone [41], and acetylcholinesterase-triggered hydrolysis of acetylthiocholine [42]. 14
(Figure 3)
3.3 Sensing of Pb2+ by interparticle crosslinking aggregation.
Because the threat of heavy-metal pollution in environmental water is still an important issue for public health, laser-induced Rayleigh scattering technique was used to detect the conditions in which metal ions act as a bridge to link ligand-modified AuNPs together. Intensive researches revealed that AuNP-based colorimetric sensors were simple, selective, and sensitive to detect heavy metal ions in environmental waters [10, 12]. This was exemplified by the use of gallic acid-capped AuNPs for probing Pb2+ in an aqueous solution [43-45]. Figure 4A illustrates that Pb2+-stimulated aggregation of gallic acid-capped AuNPs results from multivalent coordination between Pb2+ and phenolic hydroxyl groups of gallic acid [43-45]. As shown in Figure 4B, the light-scattering intensity at 532 nm of gallic acid-capped AuNPs incrementally increased with increasing the concentration of Pb2+. The calibration curve for quantifying Pb2+ was obtained by plotting the light-scattering intensity at 532 nm versus the concentrations of Pb2+ over the range from 1 to 40 nM (inset in Figure 4B). The LOD of Pb2+ was estimated to be 0.3 nM, which is superior to the LOD values measured from AuNP-based colorimetric sensors [7, 43-45], 15
nanoparticle−DNAzyme-based
colorimetric
probes
[46-49],
AuNP-based
hyper-Rayleigh scattering sensor [50], and AuNP-based dynamic light scattering sensor [51] (Table 3). Additionally, the LOD of the proposed method is much lower than the maximum level (15 ȝg L-1 ~ 72 nM) of lead in drinking water permitted by the United State Environmental Protection Agency. Figure 4C depicts that only addition of Pb2+ to a solution of gallic acid-capped AuNPs caused a dramatic change in the light-scattering intensity at 532 nm, signifying that our analytical system exhibited excellent selectivity toward Pb2+ over other metal ions. We suggest that the combination of laser-induced Rayleigh scattering technique and AuNP-based metal ion sensor could be used to probe other metal ions by modified specific ligand onto the surface of the AuNPs.
(Figure 4)
3.4 Smart-phone-based heavy metal ions reader.
To further reduce the cost of instrumentation and toward more portable system, a laser pointer (< 20 mW) and a smart-phone were substituted for a diode laser and a miniaturized spectrometer, respectively (Fig. 5A). Fig. 5B shows a representative smart-phone-captured image of solutions of gallic acid-capped AuNPs in the absence 16
and presence of 72 nM Pb2+, i.e., the safety level recommended by U. S. EPA, in drinking water. The illumination intensity of the laser pointer was sufficient to pass through the sample and control cuvettes, forming two readable signals in two image frames. To determine whether the contamination of Pb2+ in drinking water, the acquired scattering images (164 × 5 pixels) of these cuvettes were converted to pixel intensity (0−256 counts) via Image J (Figure 5C). The intensity ratio of the sample to the control cuvette was calculated by the following equation:
Ratio = (I1/A1) / (I0/A0)
I1 and I0 are the integrated pixel intensities of the selected images in the sample and control cuvettes, respectively, while A1 and A0 are the area of the selected images in the sample and control cuvette. The intensity ratio of the sample to the control cuvette was computed to be 1.5, which corresponds to the maximum allowable level of Pb2+ in drinking water. In other words, if the intensity ratio of the sample to the control cuvette is higher than 1.5, samples of drinking water will contain above 72 nM Pb2+. Thus, the combination of a laser point, a smart-phone, and a AuNP-based probe was suited to monitor the maximum allowable levels of Pb2+ in drinking water defined by the U. S. EPA..
(Figure 5) 17
4. Conclusion
We have reported a miniaturized, sensitive, and cost-effective platform for monitoring the change in light scattering from three AuNP-based sensing systems, including salt-, thiol-, and metal ions-induced aggregation of the AuNPs. Salt-induced aggregation is exemplified by the use of aptamer-modified AuNPs for sensing K+ in a high-ionic-strength solution. Laser-induced Rayleigh scattering technique coupled to aptamer-based sensor provided high sensitivity (LOD = 1 ȝM) and selectivity (21000-fold) for K+ ions. Thiol-triggered aggregation of the AuNPs is exemplified by the use of FSN-stabilized AuNPs for probing homocysteine, which is generated from SAHH-induced hydrolysis of SAH. The proposed detection system combined with FSN-stabilized AuNPs was highly responsive and selective for the SAHH/SAH system. Metal ion-promoted assembly of the AuNPs is exemplified by the detection of Pb2+ with gallic acid-capped AuNPs. By combining laser-induced Rayleigh scattering technique with gallic acid-capped AuNPs, the LOD of Pb2+ was down to 0.3 nM. Also, a sensitive, portable cost-effective smart-phone-based Pb2+ sensor platform was developed by replacing a diode laser and a miniaturized spectrometer with a laser pointer and a smart-phone. Previous studies reported that Cr3+ and Hg2+
18
triggered
the
aggregation
of
5-thiol-(2-nitrobenzoic
acid)-
and
3-mercaptopropionate acid-capped AuNPs [8, 52]. In the opinion of the authors, the smart-phone-based detection system combined with 5-thiol-(2-nitrobenzoic acid)- and 3-mercaptopropionate acid-capped AuNPs could be implemented for sensing Cr3+ and Hg2+ in an aqueous solution, respectively.
Acknowledgment
We would like to thank National Science Council (NSC 100-2628-M-110-001-MY 4) for the financial support of this study.
References
[1] J.S. Beveridge, J.R. Stephens, M.E. Williams, Annu. Rev. Anal. Chem. 4 (2011) 251. [2] D. Jariwala, V.K. Sangwan, L.J. Lauhon, T.J. Marks, M.C. Hersam, Chem. Soc. Rev. 42 (2013) 2824. [3] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Nat. Mater. 4 (2005) 435. [4] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. 19
[5] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2002) 668. [6] W.J. Luo, C.F. Zhu, S. Su, D. Li, Y. He, Q. Huang, C.H. Fan, ACS Nano 4 (2010) 7451. [7] F. Chai, C.A. Wang, T.T. Wang, L. Li, Z.M. Su, ACS Appl. Mater. Inter. 2 (2010) 1466. [8] C.J. Yu, W.L. Tseng, Langmuir 24 (2008) 12717. [9] E. Katz, I. Willner, Angew. Chem. Int. Ed. 43 (2004) 6042. [10] G. Aragay, J. Pons, A. Merkoci, Chem. Rev. 111 (2011) 3433. [11] S.J. Park, T.A. Taton, C.A. Mirkin, Science 295 (2002) 1503. [12] D. Liu, Z. Wang, X. Jiang, Nanoscale 3 (2011) 1421. [13] E. Hutter, D. Maysinger, Trends Pharmacol. Sci. 34 (2013) 497. [14] D. Vilela, M.C. Gonzalez, A. Escarpa, Anal. Chim. Acta 751 (2012) 24. [15] M. Zayats, R. Baron, I. Popov, I. Willner, Nano Lett. 5 (2005) 21. [16] R. Baron, M. Zayats, I. Willner, Anal. Chem. 77 (2005) 1566. [17] Y. Li, C. Jing, L. Zhang, Y.T. Long, Chem. Soc. Rev. 41 (2012) 632. [18] T. Bu, T. Zako, M. Fujita, M. Maeda, Chem. Commun. 49 (2013) 7531. [19] X. Liu, Q. Dai, L. Austin, J. Coutts, G. Knowles, J. Zou, H. Chen, Q. Huo, J. Am. Chem. Soc. 130 (2008) 2780. 20
[20] G.K. Darbha, A.K. Singh, U.S. Rai, E. Yu, H. Yu, P. Chandra Ray, J. Am. Chem. Soc. 130 (2008) 8038. [21] B.A. Du, Z.P. Li, C.H. Liu, Angew. Chem. Int. Ed. 45 (2006) 8022. [22]M.A. van Dijk, A.L. Tchebotareva, M. Orrit, M. Lippitz, S. Berciaud, D. Lasne, L. Cognet, B. Lounis, Phys. Chem. Chem. Phys. 8 (2006) 3486. [23] W. Zhao, M.A. Brook, Y.F. Li, Chembiochem 9 (2008) 2363. [24] L. Wang, X. Liu, X. Hu, S. Song, C. Fan, Chem. Commun. (2006) 3780. [25] H. Wei, B. Li, J. Li, E. Wang, S. Dong, Chem. Commun. (2007) 3735. [26] J. Wang, L. Wang, X. Liu, Z. Liang, S. Song, W. Li, G. Li, C. Fan, Adv. Mater. 19 (2007) 3943. [27] X. Zhu, J. Zhao, Y. Wu, Z. Shen, G. Li, Anal. Chem. 83 (2011) 4085. [28] Z.S. Wu, C.R. Chen, G.L. Shen, R.Q. Yu, Biomaterials 29 (2008) 2689. [29] C. Shi, H. Gu, C. Ma, Anal. Biochem. 400 (2010) 99. [30] D.M. Kong, J.H. Guo, W. Yang, Y.E. Ma, H.X. Shen, Biosens. Bioelectron. 25 (2009) 88. [31] S. Nagatoishi, T. Nojima, B. Juskowiak, S. Takenaka, Angew. Chem. Int. Ed. 44 (2005) 5067. [32] S. Nagatoishi, T. Nojima, E. Galezowska, A. Gluszynska, B. Juskowiak, S. Takenaka, Anal. Chim. Acta 581 (2007) 125. 21
[33] F. He, Y. Tang, S. Wang, Y. Li, D. Zhu, J. Am. Chem. Soc. 127 (2005) 12343. [34] A.E. Radi, C.K. O'Sullivan, Chem. Commun. (2006) 3432. [35] Lim, II, W. Ip, E. Crew, P.N. Njoki, D. Mott, C.J. Zhong, Y. Pan, S. Zhou, Langmuir 23 (2007) 826. [36] J.H. Lin, C.W. Chang, Z.H. Wu, W.L. Tseng, Anal. Chem. 82 (2010) 8775. [37] J.L. Palmer, R.H. Abeles, J. Biol. Chem. 254 (1979) 1217. [38] E.S. Burgos, S.A. Gulab, M.B. Cassera, V.L. Schramm, Anal. Chem. 84 (2012) 3593. [39] F. Carlucci, A. Tabucchi, A. Aiuti, F. Rosi, F. Floccari, R. Pagani, E. Marinello, Clin. Chem. 49 (2003) 1830. [40] P. Monostori, G. Wittmann, E. Karg, S. Turi, J. Chromatogr. B 877 (2009) 3331. [41] J. Perla-Kajan, H. Jakubowski, Amino Acids 43 (2012) 1405. [42] Y. Miao, N. He, J.J. Zhu, Chem. Rev. 110 (2010) 5216. [43] K.W. Huang, C.J. Yu, W.L. Tseng, Biosens. Bioelectron. 25 (2010) 984. [44] K. Yoosaf, B.I. Ipe, C.H. Suresh, K.G. Thomas, J. Phys. Chem. C 111 (2007) 12839. [45] N. Ding, Q. Cao, H. Zhao, Y. Yang, L. Zeng, Y. He, K. Xiang, G. Wang, Sensors 10 (2010) 11144. [46] J. Liu, Y. Lu, J. Am. Chem. Soc. 126 (2004) 12298. 22
[47] H. Wei, B. Li, J. Li, S. Dong, E. Wang, Nanotechnology 19 (2008) 095501. [48] D. Mazumdar, J. Liu, G. Lu, J. Zhou, Y. Lu, Chem. Commun. 46 (2010) 1416. [49] S.Y. Lin, S.H. Wu, C.H. Chen, Angew. Chem. Int. Ed. 45 (2006) 4948. [50] Y. Kim, R.C. Johnson, J.T. Hupp, Nano Lett. 1 (2001) 165. [51] L. Beqa, A.K. Singh, S.A. Khan, D. Senapati, S.R. Arumugam, P.C. Ray, ACS Appl. Mater. Interfaces 3 (2011) 668. [52] Y.J. Lai, W.L. Tseng, Analyst 136 (2011) 2712.
23
Figure 1. Scattering spectra of (A) 3.5 nm-, (B) 15 nm-, (C) 30 nm-, (D) 50 nm-sized AuNPs in the (a) absence and (b) presence of 0.5× PBS. The concentration of the AuNPs was fixed at 108 particles mL-1.
24
Figure 2. (A) Illustration of the mechanism of aptamer-modified AuNPs for sensing K+ via salt-induced NP aggregation. (B) Scattering spectra obtained from the addition of aptamer-modified AuNPs and 4−5000 ȝM K+ to 0.1× PBS (pH 7.4). The arrows indicate the signal changes with increase in K+ concentration (4 , 10, 40, 1000, 5000 ȝM). (C) Scattering intensity at 520 nm obtained from the addition of a mixture of aptamer-modified AuNPs and analyte to 0.1× PBS (pH 7.4). Analyte: (a) 10 ȝM K+, (b) 5 mM Li+, (c) 5 mM Na+, (d) 5 mM Mg2+, (e) 5 mM Ca2+, and (f) 5 mM NH4+. Metal ions were incubated with aptamer-modified AuNPs (0.75 nM) for 5 min at ambient temperature. The resulting solutions were mixed with 0.1× PBS solution for 5 min at ambient temperature.
25
Figure 3. (A) Illustration of the mechanism of FSN-stabilized AuNPs for sensing SAHH activity via thiol-induced NP aggregation. (B) Scattering spectra of FSN-stabilized AuNPs obtained after the addition of a mixture of 50 ȝM SAH and 10−50 units/L SAHH. The arrows indicate the signal changes with increase in SAHH concentration (10, 15, 20, 25, 35, 40, and 50 units/L). (C) Scattering intensity at 520 nm obtained from the addition of FSN-stabilized AuNPs to a mixture of 1.5 nM protein and 50 ȝM SAH. Protein: (a) SAHH, (b) trypsin, (c) thrombin, (d) lysozyme, (e) myoglobin, (f) cytochrome c, (g) hemoglobin, (h) Human serum albumin, and (i) bovine serum albumin. (B, C) Proteins reacted with SAH in 50 mM phosphate buffer (pH 7.2) at 37°C for 20 min. The resulting solutions were incubated with a solution containing FSN-AuNPs (0.1 nM) and phosphate buffer (100 mM, pH 5) for 10 min at ambient temperature.
26
Figure 4. (A) Illustration of the mechanism of gallic acid-capped AuNPs for sensing Pb2+ via the coordination between gallic acid and Pb2+. (B) Scattering spectra of gallic acid-capped AuNPs in the presence of increasing Pb2+ concentration. The arrows indicate the signal changes with increase in Pb2+ concentration (1, 6, 10, 20, and 40 nM). (C) Scattering intensity at 520 nm obtained from the addition of metal ion to a solution of gallic acid-capped AuNPs. Metal ion: (a) Pb2+, (b) Li+, (c) Na+, (d) K+, (e) Mg2+, (f) Ca2+, (g) Sr2+, (h) Ba2+, (i) Fe2+, (j) Fe3+, (k) Co2+, (l) Ni2+, (m) Cu2+, (n) Zn2+, (o) Mn2+, (p) Hg2+, (q) Cd2+. The concentration of Pb2+ is 10 nM, while the concentration of other metal ion is 1 ȝM. (B, C) Metal ions were incubated with a solution containing gallic acid-capped AuNPs (0.63 nM), NaClO4 (10 mM) and formic acid buffer (pH 4.5, 20 mM) for 20 min at ambient temperature.
27
Figure 5. (A) Schematic illustration of the portable device for the determination of Pb2+ in drinking water. (B) Photo images captured on the smart-phone under laser illumination. The left cuvette contained a mixture of gallic acid-capped AuNPs (0.63 nM), NaClO4 (10 mM), formic acid buffer (pH 4.5, 20 mM), and drinking water, while the right cuvette contained the above mentioned mixtures in the presence of 72 nM Pb2+. (C) Flow of image-processing steps to compute normalized pixel intensity. Conversion of selected image to integrated pixel intensity by ImageJ. The integrated pixel intensity was normalized by dividing it by the sum of pixel areas.
28
Table 1. Comparison of other AuNP-based sensors for the determination of K+ Sensing systema
detection
MDCb
LODc
Reference
aptamer-modified AuNPs
Colorimetry
1 mM
N.A.
24
Nicking endonuclease-assisted
Colorimetry
0.1 mM N.A.
27
G-quadruplex DNA-based
Electrochemical
N.A.
28
electric switch
detection
aptamer/pyrene-labeled
Fluorescence
0.6 mM 0.4 mM
29
Fluorescence
0.1 mM N.A.
30
Fluorescence
0.5 mM N.A.
31
Fluorescence
2 mM
N.A.
32
aptamer-based DNA/CCPs
Fluorescence
2 mM
N.A.
33
G-quadruplex DNA-based
Electrochemical
0.1 mM N.A.
34
electric switch
detection
aptamer-modified AuNPs
Light scattering
4 ȝM
1 ȝM
This study
AuNPs sensor N.A.
molecular beacon G-quadruplex/crystal violet complexes Pyrene-labeled G-quadruplex DNA sensor G-quadruplex DNA fluorescent sensor
a
CCPs, cationic conjugated polymers; b LDC, minimum detectable concentration.
c
N. A., not available
29
Table 2. Comparison of other methods for the determination of SAHH Sensing system
Detection
MDCa
LODb
Reference
FSN-stabilized AuNPs
colorimetry
9 nM
6 nM
36
Ellman′s reagent
Absorption
2 nM.
N.A.
36
Luciferase-based assay
fluorescence
2 nM
N.A.
39
FSN-stabilized AuNPs
light scattering
0.9 nM
0.6 nM
This study
a
MDC, minimum detectable concentration. b N. A., not available
30
Table 3. compared of other methods for the determination of Pb2+. Sensing system
Detection
MDCb
LODb
Glutathione-capped AuNPs
Colorimetry
0.1 ȝM
100 nM 7
Gallic acid-capped AuNPs
Colorimetry
10 nM
N.A.
43
Gallic acid-capped AuNPs
Colorimetry
15 ȝM
N.A.
44
Gallic acid-capped AuNPs
Colorimetry
50 nM
25 nM.
45
DNAzyme, DNA-capped
Colorimetry
0.4 ȝM
N.A.
46
Colorimetry
N.A.
500 nM 47
Colorimetry
N.A.
0.5 ȝM
48
Crown ether-modified AuNPs
Colorimetry
0.25 ȝM
N.A.
49
11-mercaptoundecanoic
HRS
N.A.
25 ȝM
50
GSH-AuNPs
DLS
50 ppt
N.A.
51
Gallic acid-capped AuNPs
Light scattering
1 nM
0.3 nM
This study
Reference
AuNPs DNAzyme, citrate-capped AuNPs DNAzyme-AuNPs dipstick test
acid-modified AuNPs
a
HRS, Hyper-Reyleigh scattering; DLS, dynamic light scattering. b N.A., not
avaliable
31
z
Laser-induced Rayleigh scattering is sensitive to detect aggregated nanoparticles.
z z
The proposed system coupled to aptamer-capped AuNPs was sensitive for K+. The proposed system coupled to FSN-stabilized AuNPs was sensitive for
z
homocysteine. The proposed system coupled to gallic acid-capped AuNPs was sensitive for Pb2+.
z
The proposed system is convenient, cheap, and portable.
32
*Graphical Abstract (for review)
A sensitive, portable cost-effective smart-phone-based Pb2+ sensor platform was developed by a laser pointer, a smart-phone, and gallic acid-capped gold nanoparticles.
