Analytica Chimica Acta 812 (2014) 114–120

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Photochemical decoration of silver nanoparticles on magnetic microspheres as substrates for the detection of adenine by surface-enhanced Raman scattering Melisew Tadele Alula, Jyisy Yang ∗ Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• We

have successfully prepared Raman active silver nanoparticles (AgNPs) on magnetic microspheres (MMs) for Raman applications. • Photochemical reduction method has been used in this work to significantly simplify the preparation procedures. • The prepared particles offer enhancement effect in Raman measurements and enrichment effect in concentration by MMs. • Prepared AgMMs have been successfully applied in determination of the important biospecies of adenine in aqueous solution.

a r t i c l e

i n f o

Article history: Received 17 July 2013 Received in revised form 22 December 2013 Accepted 24 December 2013 Available online 3 January 2014 Keywords: Adenine Magnetic microspheres Silver nanoparticles Surface-enhanced Raman scattering Photoreduction

a b s t r a c t In this work, silver nanoparticles (AgNPs) decorated magnetic microspheres (MMs) are prepared as surface-enhanced Raman scattering (SERS) substrate for the analysis of adenine in aqueous solutions. To prepare these substrates, magnetic particles were first synthesized by coprecipitation of Fe(II) and Fe(III) with ammonium hydroxide. A thin layer of cross-linked polymer was formed on these magnetic particles by polymerization through suspension of magnetic particles into a solution of divinyl benzene/methyl methacrylate. The resulted polymer protected magnetic particles are round in shape with a size of 80 ␮m in diameter. To form AgNPs on these MMs, photochemical reduction method was employed and the factors in photochemical reduction method were studied and optimized for the preparation of highly sensitive and stable AgNPs on MMs substrates (abbreviated as AgMMs substrates). By dispersing the AgMMs in aqueous samples, cylindrical magnet was used to attract the AgMMs for SERS detections. The observed enhancement factor of AgMMs reached 7 orders in magnitude for detection of adenine with a detection limit approaching to few hundreds of nanomolar. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Adenine plays a significant role in biological system as it has widespread effect to coronary and cerebral circulation, energy

∗ Corresponding author. Tel.: +886 422840411x514; fax: +886 422862547. E-mail address: [email protected] (J. Yang). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

transduction, enzymatic reactions as cofactors, and even in cell signaling [1–5]. Abnormal changes of its concentration may indicate the presence of various diseases. Quantitation of adenine is, therefore, critically needed for the studies of a wide variety of biological issues. To determine adenine, a large number of analytical methods based on electrochemistry [6–9], separation technology [10–12], colorimetric [13], fluorescence [14], Rayleigh scattering [15], and chemiluminescence [16,17] have been proposed to

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provide fast and sensitive means for detections. Alternatively, surface-enhanced Raman scattering (SERS) has started to gain attention in the analysis of nucleobases or nucleotides as the technique in applying and handling SERS substrate is gradually matured [18–25]. This technique offers the advantages of no memory effect as in electrochemical methods, no tedious pre-cleaning steps as in separation methods, and no need of labeling or enzymatic reactions as in fluorescence or colorimetric methods. In the past, magnetic nanoparticles with plasmonic properties have been reported as SERS substrate by different groups due to their nontoxic nature and ease of assembling of particles from reaction mixture [26–30]. For examples, the combination of AgNPs decorated magnetic nanoparticles with solenoid embedded microfluidic device simplifies the SERS detection [26]. The development of magnetic nanoparticles-based barcode materials enables rapid SERS determination [29]. With SERS-encoded magnetic nanoparticles, specific cancer cell can be identified by SERS measurements [30]. Surfaces of magnetic nanoparticles are usually chemically modified to limit the growth of the magnetic core and to form a coating on their surfaces for intended purposes. Surface modification also prevents aggregation and oxidation of magnetic particles. Silanization has been most commonly used for surface modification [29–32]. Copolymerization of divinyl benzene with other monomers to form cross-linked polymer has also been used to modify magnetic particles and largely used in concentration of environmental unfriendly species through the tuning of the composition in the copolymers [33–36]. The cross-linked nature of the polymer in this type of MMs provides better stability for the magnetic particles than Si based and, therefore, is selected in this work to prepare cross-linked polymer based AgMMs for decoration of AgNPs on their surfaces. The resulted polymer protected magnetic microspheres are stable because the protecting polymer layer prevents any reaction of the bare magnetic particles with surrounding chemicals and aggregation of magnetic particles themselves. Moreover, the attached silver nanoparticles (AgNPs) are not directly contacting with iron oxide, which prevents any reaction between iron oxide and AgNPs. In this work, to improve the performance of SERS detection of adenine, the concentration ability of magnetic microspheres (MMs) is integrated with the enhancement ability of AgNPs in SERS measurements. Thus, to grow AgNPs on the MMs surfaces, photochemical reduction method was modified and applied in this work. According to the literature, photochemical reduction is feasible to prepare AuNPs or AgNPs colloidal solutions [37–40]. Therefore, to assist and simplify the formation of AgNPs directly on the MMs a simple photochemical reduction method is proposed and examined. Fig. 1A shows the schematic diagram for the preparation of AgMMs in this work.

2. Experimental 2.1. Chemicals Fe(II) sulfate heptahydrate and Fe(III) sulfate n-hydrate were obtained from Showa (Tokyo, Japan). Methyl methacrylate and ammonium hydroxide (28–30% (w/v)) were purchased from Acros (Phillisburg, NJ, USA). Para-nitrothiophenol (pNTP), divinyl benzene, and ␣-␣’-azobisisobutyronitrile (AIBN) were obtained from TCI (Tokyo, Japan). Poly vinylalcohol (PVA, MW:1.24 × 105 ∼1.86 × 105 ), Polymethylmethacrylate (PMMA, MW:1.2 × 105 ) and uracil were obtained from Sigma (St. Louis, MO, USA). Methanol and toluene were obtained from Echo chemical (Toufen, Taiwan). Silver nitrate was purchased from J.T. Baker (Phillisburg, NJ, USA). Citric acid trisodium salt dehydrate was purchased from Janssen Chimica (Beerse, Belgium). Sodium


chloride was obtained from USB Corporation (Cleveland, OH, USA). Adenine, cytosine and thymine were purchased from Alfa Aesar (Ward Hill, MA, USA). Guanine was obtained from MP Biomedicals (Eschwege, Germany). Oleic acid was purchased from Wako pure chemicals (Osaka, Japan). All the chemicals were reagent grade and used without further purification. Deionized Milli-Q water was used throughout the study. 2.2. Instrumentation The Raman spectra were measured by Triax 320 Raman system (Jobin-Yvon, Inc., Longjumeau, France), equipped with 632.8 nm He/Ne laser line as excitation source (JDS Uniphase Corporation, Milpitas, CA) and a liquid-nitrogen cooled Ge array detector (Jobin-Yvon, Inc.). The laser power was 35 mW, and exposure time was 0.2 s for measurement of pNTP and 1 s for nucleobases. Scanning electron microscopy (SEM) images were obtained with JSM-6500F (JEOL, Ltd., Tokyo, Japan), field emission scanning electron microscope (FE-SEM) operating with accelerating voltage of 10 kV. UV box (TS-UV, De-Yun, Ltd., Taipei, Taiwan) operated at 30 W with a wavelength range of 320–400 nm was used as a UV light source for photo-reduction process. X-ray diffraction (XRD) patterns were obtained on a D2 phaser XRD-300 W powder diffractometer (Bruker, AXS GmbH, Karlsruhe, Germany) for a 2 range of 30–80◦ at scan rate of 0.05 degree/sec using Cu K␣ radiation at 40 kV and 100 mA. A Spectrum One FT-IR spectrometer (PerkinElmer 100 series) was used to measure the infrared spectra. Spectra were collected at a resolution of 4 cm−1 using a deuterated triglycine sulfate detector. 2.3. Preparation of polymer protected MMs To prepare MMs, magnetic particles were first prepared by coprecipitation method with some modification [36]. Briefly, 100 mL of 125 mM FeSO4 and 62.5 mM Fe2 (SO4 )3 aqueous solutions were prepared. In this solution, 10 mL of ammonium hydroxide (28–30% (w/v)) was added rapidly with vigorous stirring for 20 min to form magnetic precipitates. After addition of 4 mL oleic acid, the precipitate was kept in 85 ◦ C water bath for 1.5 h. After cooling to room temperature, the magnetic precipitates were isolated from the solvent by magnetic decantation and washed several times by deionized water. These precipitates were further washed at least two times by ethanol. 2 g of the formed magnetic precipitates were added into an organic solution containing methyl methacrylate, divinyl benzene, and toluene in volume of 10.6, 0.8, and 3 mL, respectively. In the meantime, an aqueous solution prepared by mixing 2.5 g PVA, 3 g NaCl and 100 mL deionized water was mixed with the organic solution prepared above. After ultrasonication of the mixture to form emulsion, the solution was transferred to reflux glassware. After stirring for another 30 min, 0.2 g AIBN was added to initiate the polymerization and subjected to reflux at a temperature of 70 ◦ C for 5 h. Brown magnetic emulsion was separated by magnetic decantation. After washed with deionized water and methanol several times separately, a polymer protected MMs were obtained. 2.4. Preparation of silver nanoparticles on polymer protected MMs To form AgNPs on the MMs (shortly AgMMs), 50 mg of MMs was dispersed in a 5 mL solution containing AgNO3 and trisodium citrate. After sonicating the reaction mixture for 5 min to disperse the MMs, it was placed into UV box for different lengths of time. The formed AgMMs were cleaned with deionized water and methanol several times by magnetic decantation. For the simplicity of the discussion, the conditions used for the preparation of AgMMs, were


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Fig. 1. Schematic diagram for preparation of silver coated magnetic substrate (AgMMs).

assigned as Aa Cb Ut where Aa represents a mM AgNO3 solution, Cb represents b mM citrate solution, and Ut represents t hour UV light irradiation time. 2.5. Detection procedures For pNTP detection, the AgMMs prepared from an amount of 50 mg MMs (see above) was directly used. Because of the low solubility in water, methanol was used as solvent to prepare pNTP solution. By addition of 1.5 mL of 10 ppm methanolic pNTP solution into the AgMMs bottle for 1 h, AgMMs were collected using Teflon coated 10 mm diameter cylindrical magnet and rinsed with methanol. After drying in the air, the substrates were subsequently scanned by Raman microscope as shown in Fig. 2A. For detection of adenine, 100 ␮L of 10% (w/v) AgMMs aqueous solution was added into a sample vial containing 200 ␮L of solution for 15 min. Then, a magnet was placed on the back of Teflon coated iron bar (3 mm in diameter) and subsequently inserted into the sample solution to concentrate AgMMs. In determination of adenine, samples were detected simultaneously utilizing an array vials with an array of magnetic bars as also shown in Fig. 2B. 3. Results and discussion 3.1. Basic properties of the photochemically prepared AgMMs Several properties of AgMMs are expected to ensure a sensitive SERS application. For instance, the magnetic particles should be fully covered by cross-linked polymer to protect the magnetic materials. The size of MMs should be proper to provide enough

magnetic force for concentration, while small enough to be suspended in the sample solution. The chemical properties of MMs should be able to attach AgNPs to increase the stability of the prepared AgMMs. To examine the basic properties of MMs, SERS substrate of A1.5 C0.75 U5 was first prepared and scanned by SEM as results plotted in Fig. 3. The particle size of the formed MMs was estimated from the SEM image and used to fit with a Gaussian curve. The obtained average diameter of the MMs was 79.2 ␮m with a Gaussian band width at half height of 42.1 ␮m. The average size of AgNPs on the surface of MMs was 52.6 nm with a Gaussian band width at half height of 7.5 nm. According to the tunneling electron microscope (TEM) images reported in the literature [36], the prepared microspheres show a core/shell structure as the core is composed by aggregated magnetic nanoparticles with a polymer shell. The deposition of silver on the microspheres is further confirmed by XRD patterns (refer to Fig. 3C) as four distinct diffraction peaks were observed at diffraction angles of 38.2, 44.3, 64.7, and 77.5◦ , which correspond to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystalline planes of cubic silver respectively [41]. The absence of diffraction peaks of Fe3 O4 in the XRD patterns of the AgMMs is caused by the heavy atom effect of silver, which has been reported in the literature [42,43]. To ensure that the formed polymer layer is stable under UV irradiation, MMs were irradiated with UV light for 5 h and the resulted MMs were examined by infrared (IR) spectrometry with an attenuated total reflection mode. The detected IR spectra were plotted in Fig. 4A along with none irradiated MMs and commercial PMMA. No significant difference between irradiated and none irradiated MMs can be observed revealing the UV irradiation did not degrade the polymer coating significantly. Also, comparing with the absorption bands of neat PMMA, characteristic bands of PMMA can be clearly

Fig. 2. Procedures and setup used in detections by AgMMs. The detection can be done with one substrate (A) or an array (B).

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Fig. 3. SEM images of the prepared AgMMs substrate (A1.5 C0.75 U5 ) with low magnification (A) and high magnification (B). Its corresponding XRD pattern is plotted in C.

seen in the MMs as the C O stretching band located at 1726 cm−1 and the C O related absorption bands around 1100 cm−1 . These results indicate that polymer protected MMs were successfully prepared. To characterize the new substrate for SERS activity, pNTP was used as a probe molecule. pNTP was selected as probe molecule because of its distinct Raman features and strong affinity to chemisorb on silver surfaces to form self-assembled monolayers [44,45]. To check basic properties of the prepared substrates for SERS applications, A1.5 C0.75 U5 substrates were used to detect pNTP and several 25 ␮M nucleobases as typical observed spectra are plotted in Fig. 4. The pH values were 6.1, 6.4, 6.8, and 6.9 for the solutions of adenine, cytosine, thymine, and uracil, respectively. Due to the low solubility of guanine, the pH of the guanine solution was forced to 12.4 by NaOH solution. The concentrations of the nucleobases were 25 ␮M. The detected SERS spectrum of pNTP shows three dominated bands located at 1338 cm−1 for NO2 symmetrical stretching, 1569 cm−1 for phenyl ring stretching and 1079 cm−1 for

C H bending. Band features are in agreement with the literature [44]. In the detection of nucleobases, prominent and characteristics bands are the ring breathing modes in the spectral range of 600–800 cm−1 as also can be seen in Fig. 4B. In this region, adenine has a distinguishable band located at 734 cm−1 , which is caused by ring breathing virbational mode [46]. Similar prominent bands around 796 cm−1 for cytosine, thymine, and uracil show the pyrimidine ring vibration mode [46]. These results indicate that the prepared AgMMs are suitable for SERS measurements and are feasible to determine adenine. The variation in SERS intensities could be associated with the proximity as well as the orientation of the bases that affect their coordination to the silver surfaces or the SERS cross section of the molecules. For instance, the relatively weak intensity for thymine could be associated with its weak interaction with silver nanoparticles arise from absence of exocyclic amine group that involves in interaction of cytosine with silver nanoparticles for a relatively high SERS intensity [22]. Similarly, complex formation may take in part for significantly high SERS intensity of adenine [47] in addition to its high SERS cross section [48,49]. The in-plane vibration modes of the ring in uracil may be responsible for the relatively high SERS intensity [50]. Guanine exhibits unique Raman band at 670 cm−1 and should interact similar to adenine. However, the low solubility in neutral water forces the detection under high pH, which dramatically changes the chemical system as hydroxide interacts and blocks the active sites of AgNPs.

3.2. Effect of UV irradiation time in formation of AgMMs

Fig. 4. (A) IR spectra of PMMA, MMs, and MMs with 5 h of UV irradiation. (B) SERS spectra of pNTP and nucleobases on AgMMs substrate. The concentrations of the nucleic bases are 25 ␮M and that of pNTP is 10 ppm.

To observe the processes in formation of AgNPs during irradiation, substrates of A1.5 C0.75 Ut were prepared as t varied from 0 to 6 h. During the photoreduction, the color of the solution changes from colorless to yellow readily after some minutes which shows the formation of spherical AgNPs as commonly observed in preparation of colloidal AgNPs [37]. As irradiation continues the color of the solution changes to pale green that shows the formation of more nanoparticles [51]. By probed with pNTP, the observed Raman intensity was increased to a maximal point at ca. 4 h as shown in Fig. 5A. This behavior reveals that AgNPs are gradually formed and grown to a suitable size for SERS measurements. With irradiation time longer than 5 h, the SERS intensity decreased as the size of the AgNPs is larger than needed. To verify, substrates were scanned by SEM as shown in Figs. 5B and C for those prepared with 3 and 6 h of irradiation time. Along with the SEM images shown in Fig. 3B (for 5 h of irradiation time), the size of the AgNPs is gradually increased, while the distribution density of the AgNPs is also increased with irradiation time. Interestingly, Ag nanobar starts to form with an irradiation time longer than 6 h as can be seen in Fig. 5C.


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Fig. 5. SERS intensity of pNTP on substrate of A1.5 C0.75 Ut and t varied from 0 to 9 h (A). SEM images of A1.5 C0.75 U3 (B) and A1.5 C0.75 U6 substrates (C) are also plotted.

3.3. Effect of concentration of trisodium citrate Trisodium citrate has been commonly used in production of Ag colloidal solution through wet chemical method [52–54]. This reagent works not only as a reducing agent but also as a protecting agent to cap the surface of AgNPs from serious aggregations [38]. To form suitable size of AgNPs for SERS measurements, the concentration of the citrate is usually controlled at a level lower than the concentration of silver nitrate. Moreover, high concentration of citrate may induce electrostatic repulsion among silver nanoparticles arising from its capping effect [55]. With UV radiation, in addition to its role as in wet chemical method, citrate acts as a photoactive species to photochemically release electrons to assist the reductions of silver ions [40]. To study the effect of citrate during photochemical reduction, concentration of trisodium citrate was varied while silver nitrate was kept 1, 1.5, and 2 mM in concentration. By keeping the UV irradiation time to 5 h, the prepared substrates were probed with pNTP and the observed SERS intensities for this molecule are plotted in Fig. 6A. As can be seen in this plot, Raman intensities were more pronounced in low concentration of citrate for any of the examined concentrations of silver nitrate. To understand the impact of citrate on the growth of AgNPs, substrates were scanned by SEM as plotted in Fig. 6B and C for A1.5 C0.5 U5 and A1.5 C2.0 U5 substrates, respectively. Together with the SEM image of A1.5 C0.75 U5 substrate shown in Fig. 3B, the increase of citrate concentration increases the size of AgNPs. This reveals that the capping effect did not dominate the formation of AgNPs during photoreduction. Instead, citrate plays more important role in reduction of AgNPs so that AgNPs were larger in particle size when higher concentration of citrate was used. 3.4. Effect of concentration of silver nitrate To examine the effect of silver nitrate concentration, two experiments were performed. The first experiment was performed by varying concentration of silver nitrate while the concentration of

citrate was kept at 0.75 mM. The second experiment was performed by varying the concentration of silver nitrate but the mole ratio of citrate/silver nitrate was fixed to 0.5. With an irradiation time of 5 h, the observed pNTP signals are plotted against the concentration of silver nitrate as shown in Fig. 7A. As can be seen in this figure, no matter the concentration of citrate was fixed or not, the observed relationships between pNTP band intensity and concentration of silver nitrate were similar in trend. Optimal concentration was located at 1.5 mM and further increment of silver nitrate concentration results in a significant drop of SERS signal, which could be attributed to the change of the morphology of AgNPs for higher concentration as evidenced by the SEM images in Fig. 7B and C.

3.5. Evaluation of enhancement and enrichment factor The enhancement factors (EF) were calculated using the following equation:

EF =

 I  N  SERS bulk IRaman



where the ISERS is the intensity of phenyl ring stretch of pNTP at 1569 cm−1 and the IRaman is the intensity for the same band in the Raman spectrum in 1% (w/v) methanoic solution of pNTP. The Nbulk is the number of pNTP in the laser beam path in measuring IRaman , which was calculated from the 1% (w/v) pNTP in the laser beam path (beam area × beam path). The Ndep is the number of pNTP molecules in the cross section of the laser beam, which were deposited on AgMMs with a surface density of 500 ng cm−2 . The calculated EF is found to be 4.2(±0.3) × 106 . Enrichment factor was also evaluated by comparing SERS signals obtained from dispersed and assembled AgMMs in 10 ppm pNTP solution. The obtained enrichment factor is 67(±11) indicating that AgMMs rich the analytical signal significantly with approximately 2 orders in magnitude.

Fig. 6. SERS of pNTP showing effect of concentration of trisodium citrate keeping the concentration of AgNO3 1 (),1.5 (•) and 2 mM () with UV light irradiation of 5 h (A). SEM images of the A1.5 C0.5 U5 (B) and A1.5 C2.0 U5 substrates (C).

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Fig. 7. Relationships between SERS intensity of pNTP and the concentration of silver nitrate under irradiation for 5 h (A). The concentration of citrate was kept at 0.75 mM () or the mole ratio of citrate/silver nitrate was fixed to 0.5 (•); (B) and (C) are SEM images of A0.5 C0.25 U5 and A2.5 C1.25 U5 .

3.6. Linearity, detection limit and interference study in determination of adenine To examine the linear range in SERS detection of adenine, A1.5 C0.75 U5 substrate was used as optimized from the above procedures. Also, the pronounced and isolated band of adenine located at 734 cm−1 (refer to Fig. 4) was selected for quantitative purpose. Fig. 8 shows the obtained concentration-response curve for adenine in aqueous solution. Based on this figure, SERS signal of adenine increases as concentration of adenine and remain almost constant after 20 ␮M. By fitting with Langmuir isotherm, the obtained R2 is close to 0.99. This reveals that the adsorption of adenine by AgNPs follows the surface adsorption behavior. The linear regression coefficient value is 0.991 in concentration range of 1–15 ␮M. The limit of detection was estimated on the basis of 3 times the blank test standard deviation and found to be 0.4 ␮M when the intensity at ring vibration mode is used. The enhancement factor for adenine was calculated by depositing 4 ␮L of 15 ␮M adenine solution on to AgMMs attracted on the iron bar magnet. A 1% (w/v) aqueous solution of adenine was used as bulk solution. The calculated EF was 4.2 ± 0.2 × 107 . To examine the feasibility of the AgMMs in determination of adenine under interferences of other nucleobases, adenine solutions were mixed with equimolar concentration of the other nucleic bases. The SERS intensity of adenine breathing mode at 734 cm−1 was used to determine the adenine under co-existing of second nucleobases. It was found that, the SERS intensity for adenine remains almost the same after mixed with a second nucleobases as shown in Table 1. The observed recovery, which defined as the detected SERS intensity of the adenine mixture divided by the SERS intensity for adenine alone, falls in the range of 90–102%. These

Table 1 SERS intensity of adenine at 734 cm−1 and the recoveries under different interferences. Interference

SERS intensity (Arb. Unit)


2 ␮M

10 ␮M

2 ␮Ma

10 ␮Mb

4287 (±567) 4217 (±209) 4469(±1024) 4377(±426) 4105(±271)

11852 (±567) 12120 (±457) 12120(±1004) 11449 (±401) 11309(±877)

98.5 (±13) 97.0 (±5.0) 102.6(±23.0) 100.5(±9.7) 94.0 (±6.6)

94 (±4.8) 96 (±3.8) 96 (±8.0) 91(±3.5) 90 (±7.8)


Cytosine Thymine Uracil TRISc Ammoniac


The concentration of each species in the mixture is 2 ␮M. The concentration of each species in the mixture is 10 ␮M. c The concentration of the buffer and NH3 is 10 mM regardless of the concentration of adenine. a


results indicate that our prepared SERS substrate is applicable to determine adenine even under interference of other nucleobases. 4. Conclusions In this study, we successfully applied photochemical reduction method to prepare highly stable and sensitive AgNPs decorated magnetic microspheres for SERS measurements. To improve the stability, magnetic nanoparticles were protected by a cross-linked polymer, which was formed by free radical polymerization of divinyl benzene and methyl methacrylate. The prepared AgMMs can be easily dispersed in the aqueous sample solution for rapid interaction with targeted compound. By recovering with a magnet, these concentrated AgMMs serve as SERS substrates and provide a detection limit close to few hundred nanomolar in determination of adenine. Based on the unique vibrational band of adenine located at 734 cm−1 , AgMMs prepared in this work showed selectivity in detection of adenine under several potential interference species as the recoveries were all close to 100%. Acknowledgment The authors thank the National Science Council of Republic of China for their financial support for this work. References

Fig. 8. Concentration-response profile for adenine based on A1.5 C0.75 U5 substrates. Data were also fitted with Langmuir isotherm and plotted with solid line.

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Photochemical decoration of silver nanoparticles on magnetic microspheres as substrates for the detection of adenine by surface-enhanced Raman scattering.

In this work, silver nanoparticles (AgNPs) decorated magnetic microspheres (MMs) are prepared as surface-enhanced Raman scattering (SERS) substrate fo...
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