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An organic indicator functionalized graphene oxide nanocomposite-based colorimetric assay for the detection of sarcosine
DOI: 10.1039/x0xx00000x
Zhonghua xue,*a Bo yin, a Hui Wang, a Mengqian Li, a Honghong Rao, b Xiuhui Liu, a Xinbin Zhou a
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and Xiaoquan Lu a* Rapid detection of sarcosine is a key requirement for both diagnosis and treatment of disease. We report here a simply yet sensitively colorimetric nanocomposite platform for rapid detection of sarcosine in alkaline media. The approach exploited the benefits of a rapid color-producing reaction between an organic indicator of 1,2-naphthoquinone-4-sulphonic acid sodium salt (NQS) with the analyte of sarcosine species as well as the good catalytic ability of graphene oxide (GO) to the formation of highly colored products due to its good water dispersibility, extremely large surface area and facile surface modification. As a result, NQS functionalized GO nanocomposite through π-π stacking has been demonstrated to be useful as highly efficient catalyst systems for the selective and sensitive response of colorimetric determination toward sarcosine by providing a nanocomposite-amplified colorimetric readout of the color-producing event. Meanwhile, the strategy offered excellent selectivity toward sarcosine species against other amino acids as well as a satisfying detection limit of 0.73 μM. More importantly, by using electrochemical method, a credible sensing mechanism of GO nanocomposite-based colorimetric platform for a special analyte determination can be easily verified and elucidated, which also provides an attractive alternative to conventional characterization strategies. 4
Introduction Amino acids, as key constituents of proteins, are small molecules with various functional side chain groups, which result in different roles of amino acids in physiological processes. As essential building blocks of biological molecules, they have a close relationship to life activities of biology. Therefore, the development of new methodologies for determination of amino acids is emerging as a research area of great importance because of the roles and important biological functions of amino acids in human health, 1, 2,3 diagnosis and treatment of disease. For example, sarcosine, an important exogenous amino acid, is serving in the brain as a glycine transporter type 1 (GlyT-1) inhibitor and as a source of glycine (natural coagonist of the NMDA receptor, metabolized from 4, 5,6 It was reported that the sarcosine by sarcosine dehydrogenase). mental state, overall functioning and quality of life of patients could
be improved by sarcosine adding, which is particularly important for schizophrenia patients being at metabolic risk by the disease 7-9,10 itself and medications. Additionally, sarcosine could be employed to effectively reflect the invasiveness of prostatic 11 carcinoma (Pca) and identify the growth behavior of cancer cell, which make the sarcosine become a significant biomarker for clinical diagnosis of Pca. Hence, it is of great importance to selectively detect and discriminate sarcosine as a disease-associated biomarker in biological systems. As the result of increasing attention, many efforts have been directed to the development of new methods toward amino acid analysis. So far, various conventional detection strategies such as 12, 13 14 mass spectrometry, high-performance liquid chromatography, 15, 16 17 isotope internal standard method, electrochemistry and 18 capillary zone electrophoresis have been intensively explored. However, these methods may suffer from high cost, low sensitivity and specificity, and complex operations, which made them hard to popularize further. In addition, significant interference could appear with the presence of other amino acids such as alanine and glycine. So there is still need for the development of a novel strategy for selective and sensitive determination of the sarcosine. Recently, effective and simple colorimetric sensors have been proposed and utilized for amino acids detection and recognition based on various discrimination mechanism due to their simplicity, practicality, low cost, rapid tracking of analytes, and especially that color changes 19 20, 21 can be read out by the naked eye. Such as metal complexes, 22 23, 24 hydrogen-bonds, gold/silver nanoparticles and chemical
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stacking. Similarly, the π-π stacking interaction between GO with NQS, would make a large number of NQS molecules well adsorb on both sides of the GO sheet, which may exhibit high activity in the reaction between NQS with sarcosine. To the best of our knowledge, the NQS functionalized GO materials and its application in the colorimetric determination of sarcosine has not been exploited before. In this report, we proposed utilizing the π-π stacking interaction of GO with NQS to fabricate a simple, rapid, selective, sensitive, and cost-effective colorimetric determination system toward the target of sarcosine based on direct chemical reaction between NQS with sarcosine.
Experimental section Materials Graphene oxide (GO) was obtained from Nanjing XFNANO Materials Technology Company (Raman characterization shown in Fig. S1). Sodium 1, 2-naphthoquine-4-sulfonate (NQS), all of amino acids such as sarcosine, phenylalanine, tryptophan, tyrosine, glycine, glutamic acid, aspartic acid, threonine, were supplied by Sigma Chemical Reagent Company. Other chemicals were all of analytic grade and used as received. Ultrapure water (18.25 MΩ·cm) was used in all experiments. Preparation of NQS functionalized GO nanocomposite GO was first sonicated in deionized water for 2 h to give a -1 homogeneous black solution of 2.0 mg mL . The mixture solution of NQS and GO was obtained by adding the GO solution into 2.5 mM NQS solution until the amount of GO contained 20 μg/mL GO. Sequently, the solution was vigorously stirred while maintaining for 3 h at room temperature. Finally, the solution was centrifuged for three times, and the dialysis process was further employed to obtain the homogeneous NQS functionalized GO nanocomposite (defined as NQS/GO). Characterization UV-visible absorption spectra were obtained at room temperature using a Agilent UV-8453 spectrophotometer (Agilent Inc., JPN). Scanning electron microscopy (SEM) was conducted using a Zeiss electron microscopy (Zeiss, Oberkochen, Germany) equipped with an Energy-dispersive X-ray (EDX) spectrometry (Aztec-X-80, Oxford) operated at an accelerating voltage of 5 kV. Raman spectra were taken on an Invia Raman Microscope (Renishaw) with a HeNe Laser excitation at 633 nm with a power of 5.0 mW. All electrochemical measurements and characterization were performed on a CHI660 electrochemical station (CHI Instruments Inc., USA) with a conventional three-electrode system where bare and/or modified glassy carbon electrode (GCE, 3 mm in diameter) were used as working electrode, Pt wire and Ag/AgCl (saturated KCl) as auxiliary and reference electrodes, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on a VMP2 Multi-potentiostat (Princeton Applied Research, USA) by using 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) in 0.1 M KCl solution.
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25, 26
reactions have been widely employed for visual discrimination of the targets of amino acids due to their ease of use in the solution 27 as well as the performance with high sensitivity and selectivity. Among them, because of low cost, high ratiometric response, more convenient, easy achievement and nontoxicity by avoiding steps such as modification and the utilization of some catalysts with complicated structure, chemical reactions based colorimetric assay with higher selective and sensitive have been designed and extensively applied in more and more analytical applications. In the case of sarcosine, it can be oxidized to hydrogen peroxide, formaldehyde and glycine by using sarcosine oxidase (SOD) as a catalyst, which would be helpful to fabricate indirect colorimetric sensor toward sarcosine determination by detecting the oxidation 28 reaction products mentioned above. For example, Chen et al provides a sensing platform for the label-free visual detection of sarcosine utilizing a rapid release of H2O2 by the catalytic action of sarcosine oxidase with high sensitivity and selectivity. Though it has proved to be feasible to carry on indirect quantification of sarcosine by the proposed reaction, the determination process still needs to introduce more reactants, strictly control conditions and either hard to operate. Compared to an indirect process, colorimetric assay based on the direct reaction has such features as fewer steps, simple operation, good repeatability and displays higher selectivity and stability, thus has recently attracted more and more attention. Up to now, several papers have been presented and demonstrated regarding the field of application of 1,2naphthoquinone-4-sulphonic acid sodium salt (NQS) as a chromogenic reagent for the determination of pharmaceutical 29 amines using spectrophotometry. Due to the reaction of NQS with both primary and secondary amino groups in alkaline medium, there has witnessed widespread applications of NQS as a chromogenic reagent for the determination of various 30, 31 pharmaceutical amines using spectrophotometry. Notably, NQS could be introduced as a colorimetric reagent for the determination of amino acids depends on the combination of the amino groups with NQS in an alkaline solution to form highly colored products. As far as sarcosine, to increase the interactions between NQS and the active spot of the sarcosine, some effort would be needed to introduce highly efficient catalyst systems for the sensitive response of colorimetric determination. Recent studies have demonstrated that graphene oxide (GO) have great potential for use in colorimetric sensors because of its unique characteristics such as facile surface modification, good water dispersibility, extremely large surface area, low cytotoxicity, excellent biocompatibility and others fascinating physical and chemical properties such as intrinsic 32-37 peroxidase-like activity. Moreover, the current trend of nanoscale science has been the integration of functionalized nanomaterial and technology with biology and chemistry to develop 38 new analytical tools. Inspired by the success of the aftermentioned works such as GO based colorimetric assay for 39, 40 41-43 various analytes including hydrogen peroxide, glucose , 44 45-47 48 , DNA , silver and fluoride heparin , cancer cells and biomarker 49, 50 51 52 ions, nucleotide and so on , we therefore sought to achieve enhanced signaling through the use of functionalized GO materials so as to apply them into the colorimetric applications of sarcosine detection. It has been pointed out that hemin as a flattening molecule can be assembled onto the surface of GO through π-π
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Result and discussion As well known, the direct chromogenic reaction of the indicator molecule with the analyte is one of the simplest ways to determine analyte concentration based on the formation of detectable derivatives with the naked eye. NQS, an important class of derivatizing reagents, though has its advantages and disadvantages, is still an attractive starting point in construction of colorimetric sensors toward amines and amino acids in pharmaceutical and biomedical samples, where the corresponding reaction mechanism 54, 55 can be seen in Scheme S1. As far as a colorimetric sensor, the sensitive and faster color change similar to be important and necessary for its detection applications. Based on this idea and our efforts, here, NQS and GO were employed as an indicator and response signal amplifier material to construct a simple, rapid, selective, sensitive, and cost-effective colorimetric determination assay. Scheme 1 illustrates the proposed sensoring mechanism for 56, 57 sarcosine species. As explored by literatures, NQS as a specific analytical reagent and indicator to recognize the analyte of sarcosine species, has been widely investigated. Though some researchers have pointed out that the highly colored products could be formed at a condition of heating based on the combination of the amino groups with NQS, however, in this work, a fascinating phenomena of pale orange colored compounds produced after the addition of sarcosine in the colorless solution of NQS (0.1 M NaOHH3BO3 buffer solution, pH 10.0) and importantly without any supplementary operation such as heating and stirring, yet the whole color change process is very fast and achieved with five seconds (Shown in Scheme 1A). Thus, it is supposed that the sensoring mechanism of this design strategy exploits the selectively nucleophilic substitution reaction between the amine group of sarcosine and NQS, which promotes the formation of a colored product and leads the color change from colorless to pale orange. Up to now, GO based colorimetric assay have been widely applied in the sensoring platform for detection various biomolecules by adsorbing target molecules on its surface. In this strategy, to achieve enhanced color changes and amplified colorimetric readout, NQS functionalized GO sheets were employed to make the sensor molecule of the target enriched on GO surface with high concentration to bring some positive effects on the reaction rate of the detection process. As displayed in Scheme 1, after addition of sarcosine species, the color of NQS/GO solution immediately changed from colorless to orange-yellow within a few seconds at room temperature due to a selectively and rapidly nucleophilic substitution reaction of sarcosine and NQS, strongly suggesting that the introduction of GO sheets can effectively enhance the colorproducing reaction. Notablely, the deep orange-yellow solution also can be observed after the consecutive addition of sarcosine species. Therefore, the proposed sensoring platform could be effectively employed to rapid sarcosine detection. Further, the reaction mechanism of the as-proposed strategy in Scheme 1 was confirmed by Energy-dispersive X-ray spectrometry (EDX). Firstly, GO and NQS/GO modified glassy carbon electrodes (GCE) were introduced by using dip-coated method to investigate elementary informations of materials. As we can see in Fig. 1A, the survey of GO showed the presence of detectable amout of typical C and O at 0.27 and 0.52 KeV, respectively. Compared with GO, the
detectable amount of C, O, Na, and S at 0.26, 0.51, 1.01 and 2.28 KeV was well displayed for the NQS/GO electrode, respectively, demonstrating that the noncovalent functionalization of GO by NQS successfully occurred, which rely on the good adsorption ability of GO to aromatic compound of NQS though π-π stacking interactions. Secondly, to confirm the color-producing reaction of as-prepared NQS/GO with the target of sarcosine species, a NQS/GO modified electrode was immersed into the 0.1 M NaOH-H3BO3 buffer solution (pH 10.0) containing 1 mM sarcosine at room temperature and kept for about 15 minutes. Subsequently, the EDX measurement was performed after removing the modified electrode from the solution and rinse with ultrapure water. As a result the amount of N at 0.39 KeV is expected to be clearly shown in Fig. 1C, meantime concomitant with the amount decreased of Na and S from the the intensity of 30 and 20 down to 9 and 7, respectively, strongly indicating that highly colored compounds produced as illustrated in Scheme 1. The corresponding SEM images of different samples were also provided in Fig. S2.
Scheme 1. Colorimetric detection of sarcosine with NQS (A) and NQS Functionalized GO (B)
Fig. 1 EDX spectrum of GO (a), NQS/GO without (b) and with (c) addition of 1 mM sarcosine.
With these insights, to access the feasibility of the proposed sensoring platform for sarcosine detection, the interaction of the proposed indicator molecule of NQS with the analyte of sarcosine in
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the NaOH-H3BO3 buffer solution was also verified by using UVvisible absorption spectra. No obvious peak can be observed in the absorption corresponding to the sarcosine and NQS (Fig.2a and b), respectively. However, the absorption studies of 0.1 mM NQS added with 0.1 mM sarcosine (Fig. 2c) exhibited typical reaction product absorption at 485 nm, yet corresponding color change can be clearly observed (Inset of Fig. 2), indicating the combination of the analyte of sarcosine with NQS in an alkaline solution to form highly 58, 59 colored compounds. According to the literatures, this new absorption probably is characteristic of the absorption maxima of the products of the proposed color-producing reaction based on secondary amino groups of sarcosine with NQS in alkaline medium, the corresponding reaction mechanism can be seen in Scheme S1. Therefore, it is reasonable that NQS may be an ideal choice as an indicator to construct colorimetric assay toward sarcosine determination.
electrodes. As shown in Fig. 3a, the probe of potassium ferrocyanide exhibits a clear oxidation peak at about 0.204 V with a highly current of 40.32 µA at a bare GC electrode, which was corresponding to the oxidation of Fe(CN)63- on the electrode surface. However, the modification of GO (Fig. 3b) and NQS (Fig. 3c) results in a positive shift of the potential to 0.216 V and 0.220 V, accompanying with a decrease of the current response to 18.39 µA and 15.26 µA, respectively, demonstrating that the existence of GO and/or NQS clearly decreases the current response of probe due to the adsorption of insulator species onto the GC electrode surface, suggesting the stable electron transfer process of probe and electrode surface was disturbed by insulator species such as GO and NQS modified on the electrode surface, where the oxidation 3reaction of Fe(CN)6 was inhibited and became relatively difficult. Interestingly, the introduction of NQS/GO modified GC electrode (Fig. 3d) leads to the oxidation peak of the probe shifted to the more positive potential and came at about 0.236 V, meantime, the current response further decreased to 10.44 µA. These results essentially demonstrate that the existence of NQS on the GO modified GC electrode surface leads to the decrease of electrode active area, which could inhibit the electron transfer of the probe with the electrode surface. All these results also strongly suggest that NQS could interact with GO on the GC electrode surface, where the possible mechanism may be ascribed to the π-π stacking 65 between the NQS with GO according to the reported explore.
Fig. 2 Absorption spectra of 0.1 mM NQS (a), 0.1 mM sarcosine (b) and a+b in NaOH-H3BO3 buffer solution at room temperature (c), Inset is the photograph of the corresponding solution.
In recent years, GO has been used to construct a sensing platform for detection of biomolecules by adsorbing lipophilic organic molecules on the surface. As well known, electrochemical technology was extensively applied both in construction and characterization of various electrode interfaces. Therefore, a facile yet highly believable method for rapid verifying the stacking interaction between GO with NQS and the color-producing reaction process was accomplished by using a traditional electrochemical method due to its high sensitivity and low-cost instrumentation. Importantly, compared with conventional characterization and 41,60,61 analysis methods such as HNMR, infrared spectrometry and and so on, in which sophisticated instruments, complicated processes, and long periods of time were usually involved, electrochemical technologies based on the modified electrode as one of the most important research method, have been well documented to be particularly attractive for various analytical 62,63,64 purposes due to its high efficiency and simple operation. Therefore, GO, NQS and NQS/GO modified GCE was prepared via dropping method, respectively. To investigate the interaction of GO with NQS, the electrochemical behaviour of the probe molecule at different modified electrode was investigated by electrochemical method. Fig. 3 showed the differential pulse voltammetrys (DPVs) behaviours of 1 mM K4[Fe(CN)6] containing 0.1 M KCl at different
Fig. 3 DPVs of 1.0 mM K3[Fe(CN) 6]/K4[Fe(CN)6] containing 0.1 M KCl on bare GCE, GO/GCE, NQS/GCE and NQS/GO GCE.
Electrochemical impedance spectroscopy (EIS) is an effective tool to investigate interfacial properties and processes of the 16, 55 electrode at very low overpotential. Moreover, the effective interaction of NQS with GO was also verified with impedance experiments. The electrical conductivity provided by EIS measurement is probably the best indicator of the extent to which NQS has been assembled onto the surface of GO through π-π stacking. As typical depicted in Fig. 4, both of bare and modified electrode exhibit semicircles in the high frequency region and straight line in the low frequency region, corresponding to the electron transfer process and the diffuse process, respectively. The diameter of semicircle corresponds to the interfacial electron transfer resistance (Rct), where the corresponding interfacial electron transfer resistance (Rct) of bare, GO, NQS, NQS/GO modified GCE obtained by fitting to the R(Q(RW)) equivalent circuit 39 model using the ZsimpWin program (Insert of Fig 4) are calculated
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ARTICLE effectively catalytic activity of the GO to the color-producing reaction. Beased on these results and mentioned above, it is believed that NQS functionalized GO nanocomposite to be useful as highly efficient catalyst systems for the sensitive response of colorimetric determination toward sarcosine.
Fig. 5 (A) UV−vis absorpCon spectra of (a) 0.1 mM NQS in NaOH-H3BO3 buffer solution (pH 10); (b) a + 0.1 mM sarcosine; (c) NaOH-H3BO3 buffer solution (pH 10) containing 20 μg mL-1 GO; (d) c + NQS; and (e) d + 0.1 mM sarcosine. (B) Corresponding solution colors of a, b, c, d and e.
Fig. 4 Electrochemical impedance spectroscopy measurements of 1 mM K3[Fe(CN)6]/K4 [Fe(CN)6] in 0.1 M KCl at bare GCE (black line), GO/GCE (green line), NQS/GCE (blue line) and NQS/GO GCE (red line), respectively. Inset is equivalent circuit model used to fit.
In addition, the DPVs behaviours of 1 mM K4[Fe(CN)6] at NQS/GO modified GCE with the addition of different concentration of sarcosine were also investigated. As shown in Fig. S3. With increasing the sarcosine concentration, the oxidation current of 3Fe(CN)6 decreased dramatically due to the selectively nucleophilic substitution reaction between amine group of sarcosine and NQS on the electrode surface, resulting inhibited the electron transfer of the probe with the electrode surface, which are good agreement with the results of EDX characterization presented in Fig. 1. To further assess the feasibility of NQS/GO nanocomposite for sarcosine detection, the effects of different materials on the absorption spectra and solution color change in the presence and absence of sarcosine were investigated. As we can see from Fig. 5b, after addition of sarcosine (final concentration is 0.1 mM) to the colorless solution of 0.1 M NaOH-H3BO3 containing 0.1 mM NQS (Fig. 5a), the color of resulting solution immediately turns to pale orange with maximum absorbance at 485 nm due to the production of colored compounds. Especially, the color change of the solution at room temperature is very fast and without any supplementary operation such as heating and stirring, suggesting that a characteristic color change in the visible range could be employed for the sensitive discrimination of the added sarcosine sample. Moreover, it is well known that the high specific surface area makes GO a promising material for a catalyzing reaction. As displayed in Fig. 5e, for the colorless solution of NQS/GO, the orange-yellow product with a maximum absorption wavelength of 485 nm was further achieved when the addition of sarcosine, demonstrating an
To further investigate the effect of pH and GO on the sensoring assay, the amount of GO and pH need to be optimized so that their influence on the sensitivity of the sensing process will be minimal. As depicted in Fig. 6A, the absorption value at 485 nm increased with the increase of pH from 7.0 to 10.0 until it reached a plateau, followed decreases from 10.0 to 12.0. Meantime, as shown in Fig. S4, with the increase of pH until 10.0 achieved, the solution employed here was notably changed from colorless to deep orangeyellow. Therefore, pH 10.0 was used for subsequent detection. Additionally, sensitively activity of the sensoring assay is also depending on the amount of GO in the proposed NQS/GO materials. As shown in Fig. 6B, the addition of GO resulted an obvious increase of absorption value, indicating GO can effectively -1 promote the color-producing reaction. Clearly, 20 μg mL GO was chosen as the optimal amount for the detection of sarcosine.
Fig. 6 Dependence of the absorbance value of the as-gained colored compound at 485 nm on pH (A) and the concentration of GO (B).
To demonstrate the selectivity and applicability of the proposed approach for quantitative detection of sarcosine, the absorption spectra of NQS functionalized GO nanocomposite at varied solutions containing glutamic acid, tyrosine, phenylalanine, tryptophan, glycine, aspartic acid, threonine and sarcosine, respectively, was further investigated. As depicted in Fig. 7A, the characteristic remarkable absorption peak at 485 nm was found only in NQS/GO holding sarcosine solution while more weak absorption for other amino acids mentioned above. Moreover, it is obvious that only sarcosine produced a colorimetric response of colour change from colourless to orange-yellow (Inset of Fig. 7), indicating that the proposed materials of NQS/GO could selectively
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to be ~116, 217, 262 and 301 Ω, respectively. Moreover, the apparent electron transfer (ET) rate constant (kapp) according to the 66 -7 -7 reported method are calculated to be ~4.59×10 , 2.45×10 , -7 -7 -1 2.03×10 , 1.77×10 cm s (Table S1). This indicates that NQS was adsorbed and assembled on the GO modified GCE surface, resulting a lower kapp of the NQS/GO modified GC electrode was well presented. Obviously, DPVs and EIS experiment results again demonstrate that the stable adsorption of NQS with GO would indeed happen, which could be ascribed to the π-π stacking interaction between GO with NQS.
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target sarcosine species, which may be due to the highly special affinity between sarcosine and NQS. It also can be clearly seen from the competition experiments displayed in Fig. 7B. As expected, absorption responses of NQS/GO toward sarcosine were significantly higher than other amino acids, demonstrating amino acids mentioned above did not interfere with the higherly selective sensing system of NQS/GO for sarcosine, which usually is desired for sensitive colorimetric sensors.
proposed NQS/GO nanocomposite has been successfully applied into colorimetric determination toward the target of sarcosine and clearly demonstrated to be potentially useful for accelerating the color-producing reaction as well as with good solubility and stability in the analyte detectiion process. More importantly, this approach provides a new design strategy for developing GO based colorimetric platform to make the watersoluble sensors detect the components in aqueous media. Additionally, we also demonstrate here a feasible and facile method for verifing the feasibility of the proposed sensing platform for sarcosine detection by employing a simple yet effectively electrochemical methods, which will open a new avenue for developing colorimetric detection methodologies due to its great scientific and practical values.
Acknowledgements Fig. 7 (A) UV−vis absorpCon spectra of NQS/GO solution (NQS/GO= 2.5 mM/20 µg mL-1) with the addition of 0.1 mM glutamic acid (Glu), tyrosine (Tyr), phenylalanine (Phe), tryptophan (Try), glycine (Gly), aspartic acid (Asp), threonine (Thr) and sarcosine (Sar), respectively. Inset is the corresponding solution colors. (B) Absorbance enhancement (A− A0) at 485 nm for different amino acids (0.1 mM) in the absence and presence of NQS/GO. Here, A0 and A is the absorbance of amino acids in the absence and presence of NQS/GO, respectively.
This work is supported by the National Natural Science Foundation of China (no. 21265009, 21265018, 21565021), Program for Longyuan Innovative Research Talent of Gansu, China, Research Fund for the Doctoral Program of Higher Education of China (20126203120003) and Programfor Chang Jiang Scholars and Innovative Research Team, Ministry of Education, China (Grant no. IRT1283).
Notes and references 1. 2.
Fig. 8 (A) UV−vis spectra of NQS/GO sensing system with different concentrations of sarcosine, Inset is the corresponding solution colors; (B) Calibration response for the determination of sarcosine. Concentrations range from 0, 6.2, 10.5, 16.4, 26.3, 52.6, 105, 158 to 263 μM.
Under the optimized conditions, the UV-vis absorption spectra of proposed materials of NQS/GO in the presence of sarcosine with different concentrations were recorded. Fig 8A illustrated that the absorbance at 485 nm gradually increased with increasing concentration of sarcosine. Analogous optical responses with notable color changes were also gained and shown (Inset of Fig. 8A), strongly supporting the feasibility and applicability of the potential applications of the proposed colorimetric system. The absorption values exhibited a good linear correlation to the concentration of 2 sarcosine in the range of 6.2 ~ 26.3 μM (R = 0.9974) and 26.3 ~ 263 2 μM (R = 0.9946) shown in Fig 8B. Based on 3σ/s (σ is the standard deviation of the blank measurements and s is the sensitivity of the calibration graph), the detection limit of sarcosine was calculated to be 0.73 μM. Such performance characteristics suggested a potential use of as-proposed colorimetric platform for sarcosine sensing.
3. 4. 5. 6.
7. 8.
9. 10. 11.
Conclusions
12.
In summary, a simple and rapid colorimetric nanosensor with high selectivity and sensitivity based on NQS functionalized GO nanocomposite through π-π stacking was explored. The
13.
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An organic indicator functionalized graphene oxide nanocomposite-based colorimetric assay for the detection of sarcosine
DOI: 10.1039/x0xx00000x
Zhonghua xue,*a Bo yin, a Hui Wang, a Mengqian Li, a Honghong Rao, b Xiuhui Liu, a Xinbin Zhou a
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and Xiaoquan Lu a* Rapid detection of sarcosine is a key requirement for both diagnosis and treatment of disease. We report here a simply yet sensitively colorimetric nanocomposite platform for rapid detection of sarcosine in alkaline media. The approach exploited the benefits of a rapid color-producing reaction between an organic indicator of 1,2-naphthoquinone-4-sulphonic acid sodium salt (NQS) with the analyte of sarcosine species as well as the good catalytic ability of graphene oxide (GO) to the formation of highly colored products due to its good water dispersibility, extremely large surface area and facile surface modification. As a result, NQS functionalized GO nanocomposite through π-π stacking has been demonstrated to be useful as highly efficient catalyst systems for the selective and sensitive response of colorimetric determination toward sarcosine by providing a nanocomposite-amplified colorimetric readout of the color-producing event. Meanwhile, the strategy offered excellent selectivity toward sarcosine species against other amino acids as well as a satisfying detection limit of 0.73 μM. More importantly, by using electrochemical method, a credible sensing mechanism of GO nanocomposite-based colorimetric platform for a special analyte determination can be easily verified and elucidated, which also provides an attractive alternative to conventional characterization strategies. 4
Introduction Amino acids, as key constituents of proteins, are small molecules with various functional side chain groups, which result in different roles of amino acids in physiological processes. As essential building blocks of biological molecules, they have a close relationship to life activities of biology. Therefore, the development of new methodologies for determination of amino acids is emerging as a research area of great importance because of the roles and important biological functions of amino acids in human health, 1, 2,3 diagnosis and treatment of disease. For example, sarcosine, an important exogenous amino acid, is serving in the brain as a glycine transporter type 1 (GlyT-1) inhibitor and as a source of glycine (natural coagonist of the NMDA receptor, metabolized from 4, 5,6 It was reported that the sarcosine by sarcosine dehydrogenase). mental state, overall functioning and quality of life of patients could
be improved by sarcosine adding, which is particularly important for schizophrenia patients being at metabolic risk by the disease 7-9,10 itself and medications. Additionally, sarcosine could be employed to effectively reflect the invasiveness of prostatic 11 carcinoma (Pca) and identify the growth behavior of cancer cell, which make the sarcosine become a significant biomarker for clinical diagnosis of Pca. Hence, it is of great importance to selectively detect and discriminate sarcosine as a disease-associated biomarker in biological systems. As the result of increasing attention, many efforts have been directed to the development of new methods toward amino acid analysis. So far, various conventional detection strategies such as 12, 13 14 mass spectrometry, high-performance liquid chromatography, 15, 16 17 isotope internal standard method, electrochemistry and 18 capillary zone electrophoresis have been intensively explored. However, these methods may suffer from high cost, low sensitivity and specificity, and complex operations, which made them hard to popularize further. In addition, significant interference could appear with the presence of other amino acids such as alanine and glycine. So there is still need for the development of a novel strategy for selective and sensitive determination of the sarcosine. Recently, effective and simple colorimetric sensors have been proposed and utilized for amino acids detection and recognition based on various discrimination mechanism due to their simplicity, practicality, low cost, rapid tracking of analytes, and especially that color changes 19 20, 21 can be read out by the naked eye. Such as metal complexes, 22 23, 24 hydrogen-bonds, gold/silver nanoparticles and chemical
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stacking. Similarly, the π-π stacking interaction between GO with NQS, would make a large number of NQS molecules well adsorb on both sides of the GO sheet, which may exhibit high activity in the reaction between NQS with sarcosine. To the best of our knowledge, the NQS functionalized GO materials and its application in the colorimetric determination of sarcosine has not been exploited before. In this report, we proposed utilizing the π-π stacking interaction of GO with NQS to fabricate a simple, rapid, selective, sensitive, and cost-effective colorimetric determination system toward the target of sarcosine based on direct chemical reaction between NQS with sarcosine.
Experimental section Materials Graphene oxide (GO) was obtained from Nanjing XFNANO Materials Technology Company (Raman characterization shown in Fig. S1). Sodium 1, 2-naphthoquine-4-sulfonate (NQS), all of amino acids such as sarcosine, phenylalanine, tryptophan, tyrosine, glycine, glutamic acid, aspartic acid, threonine, were supplied by Sigma Chemical Reagent Company. Other chemicals were all of analytic grade and used as received. Ultrapure water (18.25 MΩ·cm) was used in all experiments. Preparation of NQS functionalized GO nanocomposite GO was first sonicated in deionized water for 2 h to give a -1 homogeneous black solution of 2.0 mg mL . The mixture solution of NQS and GO was obtained by adding the GO solution into 2.5 mM NQS solution until the amount of GO contained 20 μg/mL GO. Sequently, the solution was vigorously stirred while maintaining for 3 h at room temperature. Finally, the solution was centrifuged for three times, and the dialysis process was further employed to obtain the homogeneous NQS functionalized GO nanocomposite (defined as NQS/GO). Characterization UV-visible absorption spectra were obtained at room temperature using a Agilent UV-8453 spectrophotometer (Agilent Inc., JPN). Scanning electron microscopy (SEM) was conducted using a Zeiss electron microscopy (Zeiss, Oberkochen, Germany) equipped with an Energy-dispersive X-ray (EDX) spectrometry (Aztec-X-80, Oxford) operated at an accelerating voltage of 5 kV. Raman spectra were taken on an Invia Raman Microscope (Renishaw) with a HeNe Laser excitation at 633 nm with a power of 5.0 mW. All electrochemical measurements and characterization were performed on a CHI660 electrochemical station (CHI Instruments Inc., USA) with a conventional three-electrode system where bare and/or modified glassy carbon electrode (GCE, 3 mm in diameter) were used as working electrode, Pt wire and Ag/AgCl (saturated KCl) as auxiliary and reference electrodes, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on a VMP2 Multi-potentiostat (Princeton Applied Research, USA) by using 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) in 0.1 M KCl solution.
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reactions have been widely employed for visual discrimination of the targets of amino acids due to their ease of use in the solution 27 as well as the performance with high sensitivity and selectivity. Among them, because of low cost, high ratiometric response, more convenient, easy achievement and nontoxicity by avoiding steps such as modification and the utilization of some catalysts with complicated structure, chemical reactions based colorimetric assay with higher selective and sensitive have been designed and extensively applied in more and more analytical applications. In the case of sarcosine, it can be oxidized to hydrogen peroxide, formaldehyde and glycine by using sarcosine oxidase (SOD) as a catalyst, which would be helpful to fabricate indirect colorimetric sensor toward sarcosine determination by detecting the oxidation 28 reaction products mentioned above. For example, Chen et al provides a sensing platform for the label-free visual detection of sarcosine utilizing a rapid release of H2O2 by the catalytic action of sarcosine oxidase with high sensitivity and selectivity. Though it has proved to be feasible to carry on indirect quantification of sarcosine by the proposed reaction, the determination process still needs to introduce more reactants, strictly control conditions and either hard to operate. Compared to an indirect process, colorimetric assay based on the direct reaction has such features as fewer steps, simple operation, good repeatability and displays higher selectivity and stability, thus has recently attracted more and more attention. Up to now, several papers have been presented and demonstrated regarding the field of application of 1,2naphthoquinone-4-sulphonic acid sodium salt (NQS) as a chromogenic reagent for the determination of pharmaceutical 29 amines using spectrophotometry. Due to the reaction of NQS with both primary and secondary amino groups in alkaline medium, there has witnessed widespread applications of NQS as a chromogenic reagent for the determination of various 30, 31 pharmaceutical amines using spectrophotometry. Notably, NQS could be introduced as a colorimetric reagent for the determination of amino acids depends on the combination of the amino groups with NQS in an alkaline solution to form highly colored products. As far as sarcosine, to increase the interactions between NQS and the active spot of the sarcosine, some effort would be needed to introduce highly efficient catalyst systems for the sensitive response of colorimetric determination. Recent studies have demonstrated that graphene oxide (GO) have great potential for use in colorimetric sensors because of its unique characteristics such as facile surface modification, good water dispersibility, extremely large surface area, low cytotoxicity, excellent biocompatibility and others fascinating physical and chemical properties such as intrinsic 32-37 peroxidase-like activity. Moreover, the current trend of nanoscale science has been the integration of functionalized nanomaterial and technology with biology and chemistry to develop 38 new analytical tools. Inspired by the success of the aftermentioned works such as GO based colorimetric assay for 39, 40 41-43 various analytes including hydrogen peroxide, glucose , 44 45-47 48 , DNA , silver and fluoride heparin , cancer cells and biomarker 49, 50 51 52 ions, nucleotide and so on , we therefore sought to achieve enhanced signaling through the use of functionalized GO materials so as to apply them into the colorimetric applications of sarcosine detection. It has been pointed out that hemin as a flattening molecule can be assembled onto the surface of GO through π-π
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Result and discussion As well known, the direct chromogenic reaction of the indicator molecule with the analyte is one of the simplest ways to determine analyte concentration based on the formation of detectable derivatives with the naked eye. NQS, an important class of derivatizing reagents, though has its advantages and disadvantages, is still an attractive starting point in construction of colorimetric sensors toward amines and amino acids in pharmaceutical and biomedical samples, where the corresponding reaction mechanism 54, 55 can be seen in Scheme S1. As far as a colorimetric sensor, the sensitive and faster color change similar to be important and necessary for its detection applications. Based on this idea and our efforts, here, NQS and GO were employed as an indicator and response signal amplifier material to construct a simple, rapid, selective, sensitive, and cost-effective colorimetric determination assay. Scheme 1 illustrates the proposed sensoring mechanism for 56, 57 sarcosine species. As explored by literatures, NQS as a specific analytical reagent and indicator to recognize the analyte of sarcosine species, has been widely investigated. Though some researchers have pointed out that the highly colored products could be formed at a condition of heating based on the combination of the amino groups with NQS, however, in this work, a fascinating phenomena of pale orange colored compounds produced after the addition of sarcosine in the colorless solution of NQS (0.1 M NaOHH3BO3 buffer solution, pH 10.0) and importantly without any supplementary operation such as heating and stirring, yet the whole color change process is very fast and achieved with five seconds (Shown in Scheme 1A). Thus, it is supposed that the sensoring mechanism of this design strategy exploits the selectively nucleophilic substitution reaction between the amine group of sarcosine and NQS, which promotes the formation of a colored product and leads the color change from colorless to pale orange. Up to now, GO based colorimetric assay have been widely applied in the sensoring platform for detection various biomolecules by adsorbing target molecules on its surface. In this strategy, to achieve enhanced color changes and amplified colorimetric readout, NQS functionalized GO sheets were employed to make the sensor molecule of the target enriched on GO surface with high concentration to bring some positive effects on the reaction rate of the detection process. As displayed in Scheme 1, after addition of sarcosine species, the color of NQS/GO solution immediately changed from colorless to orange-yellow within a few seconds at room temperature due to a selectively and rapidly nucleophilic substitution reaction of sarcosine and NQS, strongly suggesting that the introduction of GO sheets can effectively enhance the colorproducing reaction. Notablely, the deep orange-yellow solution also can be observed after the consecutive addition of sarcosine species. Therefore, the proposed sensoring platform could be effectively employed to rapid sarcosine detection. Further, the reaction mechanism of the as-proposed strategy in Scheme 1 was confirmed by Energy-dispersive X-ray spectrometry (EDX). Firstly, GO and NQS/GO modified glassy carbon electrodes (GCE) were introduced by using dip-coated method to investigate elementary informations of materials. As we can see in Fig. 1A, the survey of GO showed the presence of detectable amout of typical C and O at 0.27 and 0.52 KeV, respectively. Compared with GO, the
detectable amount of C, O, Na, and S at 0.26, 0.51, 1.01 and 2.28 KeV was well displayed for the NQS/GO electrode, respectively, demonstrating that the noncovalent functionalization of GO by NQS successfully occurred, which rely on the good adsorption ability of GO to aromatic compound of NQS though π-π stacking interactions. Secondly, to confirm the color-producing reaction of as-prepared NQS/GO with the target of sarcosine species, a NQS/GO modified electrode was immersed into the 0.1 M NaOH-H3BO3 buffer solution (pH 10.0) containing 1 mM sarcosine at room temperature and kept for about 15 minutes. Subsequently, the EDX measurement was performed after removing the modified electrode from the solution and rinse with ultrapure water. As a result the amount of N at 0.39 KeV is expected to be clearly shown in Fig. 1C, meantime concomitant with the amount decreased of Na and S from the the intensity of 30 and 20 down to 9 and 7, respectively, strongly indicating that highly colored compounds produced as illustrated in Scheme 1. The corresponding SEM images of different samples were also provided in Fig. S2.
Scheme 1. Colorimetric detection of sarcosine with NQS (A) and NQS Functionalized GO (B)
Fig. 1 EDX spectrum of GO (a), NQS/GO without (b) and with (c) addition of 1 mM sarcosine.
With these insights, to access the feasibility of the proposed sensoring platform for sarcosine detection, the interaction of the proposed indicator molecule of NQS with the analyte of sarcosine in
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the NaOH-H3BO3 buffer solution was also verified by using UVvisible absorption spectra. No obvious peak can be observed in the absorption corresponding to the sarcosine and NQS (Fig.2a and b), respectively. However, the absorption studies of 0.1 mM NQS added with 0.1 mM sarcosine (Fig. 2c) exhibited typical reaction product absorption at 485 nm, yet corresponding color change can be clearly observed (Inset of Fig. 2), indicating the combination of the analyte of sarcosine with NQS in an alkaline solution to form highly 58, 59 colored compounds. According to the literatures, this new absorption probably is characteristic of the absorption maxima of the products of the proposed color-producing reaction based on secondary amino groups of sarcosine with NQS in alkaline medium, the corresponding reaction mechanism can be seen in Scheme S1. Therefore, it is reasonable that NQS may be an ideal choice as an indicator to construct colorimetric assay toward sarcosine determination.
electrodes. As shown in Fig. 3a, the probe of potassium ferrocyanide exhibits a clear oxidation peak at about 0.204 V with a highly current of 40.32 µA at a bare GC electrode, which was corresponding to the oxidation of Fe(CN)63- on the electrode surface. However, the modification of GO (Fig. 3b) and NQS (Fig. 3c) results in a positive shift of the potential to 0.216 V and 0.220 V, accompanying with a decrease of the current response to 18.39 µA and 15.26 µA, respectively, demonstrating that the existence of GO and/or NQS clearly decreases the current response of probe due to the adsorption of insulator species onto the GC electrode surface, suggesting the stable electron transfer process of probe and electrode surface was disturbed by insulator species such as GO and NQS modified on the electrode surface, where the oxidation 3reaction of Fe(CN)6 was inhibited and became relatively difficult. Interestingly, the introduction of NQS/GO modified GC electrode (Fig. 3d) leads to the oxidation peak of the probe shifted to the more positive potential and came at about 0.236 V, meantime, the current response further decreased to 10.44 µA. These results essentially demonstrate that the existence of NQS on the GO modified GC electrode surface leads to the decrease of electrode active area, which could inhibit the electron transfer of the probe with the electrode surface. All these results also strongly suggest that NQS could interact with GO on the GC electrode surface, where the possible mechanism may be ascribed to the π-π stacking 65 between the NQS with GO according to the reported explore.
Fig. 2 Absorption spectra of 0.1 mM NQS (a), 0.1 mM sarcosine (b) and a+b in NaOH-H3BO3 buffer solution at room temperature (c), Inset is the photograph of the corresponding solution.
In recent years, GO has been used to construct a sensing platform for detection of biomolecules by adsorbing lipophilic organic molecules on the surface. As well known, electrochemical technology was extensively applied both in construction and characterization of various electrode interfaces. Therefore, a facile yet highly believable method for rapid verifying the stacking interaction between GO with NQS and the color-producing reaction process was accomplished by using a traditional electrochemical method due to its high sensitivity and low-cost instrumentation. Importantly, compared with conventional characterization and 41,60,61 analysis methods such as HNMR, infrared spectrometry and and so on, in which sophisticated instruments, complicated processes, and long periods of time were usually involved, electrochemical technologies based on the modified electrode as one of the most important research method, have been well documented to be particularly attractive for various analytical 62,63,64 purposes due to its high efficiency and simple operation. Therefore, GO, NQS and NQS/GO modified GCE was prepared via dropping method, respectively. To investigate the interaction of GO with NQS, the electrochemical behaviour of the probe molecule at different modified electrode was investigated by electrochemical method. Fig. 3 showed the differential pulse voltammetrys (DPVs) behaviours of 1 mM K4[Fe(CN)6] containing 0.1 M KCl at different
Fig. 3 DPVs of 1.0 mM K3[Fe(CN) 6]/K4[Fe(CN)6] containing 0.1 M KCl on bare GCE, GO/GCE, NQS/GCE and NQS/GO GCE.
Electrochemical impedance spectroscopy (EIS) is an effective tool to investigate interfacial properties and processes of the 16, 55 electrode at very low overpotential. Moreover, the effective interaction of NQS with GO was also verified with impedance experiments. The electrical conductivity provided by EIS measurement is probably the best indicator of the extent to which NQS has been assembled onto the surface of GO through π-π stacking. As typical depicted in Fig. 4, both of bare and modified electrode exhibit semicircles in the high frequency region and straight line in the low frequency region, corresponding to the electron transfer process and the diffuse process, respectively. The diameter of semicircle corresponds to the interfacial electron transfer resistance (Rct), where the corresponding interfacial electron transfer resistance (Rct) of bare, GO, NQS, NQS/GO modified GCE obtained by fitting to the R(Q(RW)) equivalent circuit 39 model using the ZsimpWin program (Insert of Fig 4) are calculated
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ARTICLE effectively catalytic activity of the GO to the color-producing reaction. Beased on these results and mentioned above, it is believed that NQS functionalized GO nanocomposite to be useful as highly efficient catalyst systems for the sensitive response of colorimetric determination toward sarcosine.
Fig. 5 (A) UV−vis absorpCon spectra of (a) 0.1 mM NQS in NaOH-H3BO3 buffer solution (pH 10); (b) a + 0.1 mM sarcosine; (c) NaOH-H3BO3 buffer solution (pH 10) containing 20 μg mL-1 GO; (d) c + NQS; and (e) d + 0.1 mM sarcosine. (B) Corresponding solution colors of a, b, c, d and e.
Fig. 4 Electrochemical impedance spectroscopy measurements of 1 mM K3[Fe(CN)6]/K4 [Fe(CN)6] in 0.1 M KCl at bare GCE (black line), GO/GCE (green line), NQS/GCE (blue line) and NQS/GO GCE (red line), respectively. Inset is equivalent circuit model used to fit.
In addition, the DPVs behaviours of 1 mM K4[Fe(CN)6] at NQS/GO modified GCE with the addition of different concentration of sarcosine were also investigated. As shown in Fig. S3. With increasing the sarcosine concentration, the oxidation current of 3Fe(CN)6 decreased dramatically due to the selectively nucleophilic substitution reaction between amine group of sarcosine and NQS on the electrode surface, resulting inhibited the electron transfer of the probe with the electrode surface, which are good agreement with the results of EDX characterization presented in Fig. 1. To further assess the feasibility of NQS/GO nanocomposite for sarcosine detection, the effects of different materials on the absorption spectra and solution color change in the presence and absence of sarcosine were investigated. As we can see from Fig. 5b, after addition of sarcosine (final concentration is 0.1 mM) to the colorless solution of 0.1 M NaOH-H3BO3 containing 0.1 mM NQS (Fig. 5a), the color of resulting solution immediately turns to pale orange with maximum absorbance at 485 nm due to the production of colored compounds. Especially, the color change of the solution at room temperature is very fast and without any supplementary operation such as heating and stirring, suggesting that a characteristic color change in the visible range could be employed for the sensitive discrimination of the added sarcosine sample. Moreover, it is well known that the high specific surface area makes GO a promising material for a catalyzing reaction. As displayed in Fig. 5e, for the colorless solution of NQS/GO, the orange-yellow product with a maximum absorption wavelength of 485 nm was further achieved when the addition of sarcosine, demonstrating an
To further investigate the effect of pH and GO on the sensoring assay, the amount of GO and pH need to be optimized so that their influence on the sensitivity of the sensing process will be minimal. As depicted in Fig. 6A, the absorption value at 485 nm increased with the increase of pH from 7.0 to 10.0 until it reached a plateau, followed decreases from 10.0 to 12.0. Meantime, as shown in Fig. S4, with the increase of pH until 10.0 achieved, the solution employed here was notably changed from colorless to deep orangeyellow. Therefore, pH 10.0 was used for subsequent detection. Additionally, sensitively activity of the sensoring assay is also depending on the amount of GO in the proposed NQS/GO materials. As shown in Fig. 6B, the addition of GO resulted an obvious increase of absorption value, indicating GO can effectively -1 promote the color-producing reaction. Clearly, 20 μg mL GO was chosen as the optimal amount for the detection of sarcosine.
Fig. 6 Dependence of the absorbance value of the as-gained colored compound at 485 nm on pH (A) and the concentration of GO (B).
To demonstrate the selectivity and applicability of the proposed approach for quantitative detection of sarcosine, the absorption spectra of NQS functionalized GO nanocomposite at varied solutions containing glutamic acid, tyrosine, phenylalanine, tryptophan, glycine, aspartic acid, threonine and sarcosine, respectively, was further investigated. As depicted in Fig. 7A, the characteristic remarkable absorption peak at 485 nm was found only in NQS/GO holding sarcosine solution while more weak absorption for other amino acids mentioned above. Moreover, it is obvious that only sarcosine produced a colorimetric response of colour change from colourless to orange-yellow (Inset of Fig. 7), indicating that the proposed materials of NQS/GO could selectively
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to be ~116, 217, 262 and 301 Ω, respectively. Moreover, the apparent electron transfer (ET) rate constant (kapp) according to the 66 -7 -7 reported method are calculated to be ~4.59×10 , 2.45×10 , -7 -7 -1 2.03×10 , 1.77×10 cm s (Table S1). This indicates that NQS was adsorbed and assembled on the GO modified GCE surface, resulting a lower kapp of the NQS/GO modified GC electrode was well presented. Obviously, DPVs and EIS experiment results again demonstrate that the stable adsorption of NQS with GO would indeed happen, which could be ascribed to the π-π stacking interaction between GO with NQS.
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target sarcosine species, which may be due to the highly special affinity between sarcosine and NQS. It also can be clearly seen from the competition experiments displayed in Fig. 7B. As expected, absorption responses of NQS/GO toward sarcosine were significantly higher than other amino acids, demonstrating amino acids mentioned above did not interfere with the higherly selective sensing system of NQS/GO for sarcosine, which usually is desired for sensitive colorimetric sensors.
proposed NQS/GO nanocomposite has been successfully applied into colorimetric determination toward the target of sarcosine and clearly demonstrated to be potentially useful for accelerating the color-producing reaction as well as with good solubility and stability in the analyte detectiion process. More importantly, this approach provides a new design strategy for developing GO based colorimetric platform to make the watersoluble sensors detect the components in aqueous media. Additionally, we also demonstrate here a feasible and facile method for verifing the feasibility of the proposed sensing platform for sarcosine detection by employing a simple yet effectively electrochemical methods, which will open a new avenue for developing colorimetric detection methodologies due to its great scientific and practical values.
Acknowledgements Fig. 7 (A) UV−vis absorpCon spectra of NQS/GO solution (NQS/GO= 2.5 mM/20 µg mL-1) with the addition of 0.1 mM glutamic acid (Glu), tyrosine (Tyr), phenylalanine (Phe), tryptophan (Try), glycine (Gly), aspartic acid (Asp), threonine (Thr) and sarcosine (Sar), respectively. Inset is the corresponding solution colors. (B) Absorbance enhancement (A− A0) at 485 nm for different amino acids (0.1 mM) in the absence and presence of NQS/GO. Here, A0 and A is the absorbance of amino acids in the absence and presence of NQS/GO, respectively.
This work is supported by the National Natural Science Foundation of China (no. 21265009, 21265018, 21565021), Program for Longyuan Innovative Research Talent of Gansu, China, Research Fund for the Doctoral Program of Higher Education of China (20126203120003) and Programfor Chang Jiang Scholars and Innovative Research Team, Ministry of Education, China (Grant no. IRT1283).
Notes and references 1. 2.
Fig. 8 (A) UV−vis spectra of NQS/GO sensing system with different concentrations of sarcosine, Inset is the corresponding solution colors; (B) Calibration response for the determination of sarcosine. Concentrations range from 0, 6.2, 10.5, 16.4, 26.3, 52.6, 105, 158 to 263 μM.
Under the optimized conditions, the UV-vis absorption spectra of proposed materials of NQS/GO in the presence of sarcosine with different concentrations were recorded. Fig 8A illustrated that the absorbance at 485 nm gradually increased with increasing concentration of sarcosine. Analogous optical responses with notable color changes were also gained and shown (Inset of Fig. 8A), strongly supporting the feasibility and applicability of the potential applications of the proposed colorimetric system. The absorption values exhibited a good linear correlation to the concentration of 2 sarcosine in the range of 6.2 ~ 26.3 μM (R = 0.9974) and 26.3 ~ 263 2 μM (R = 0.9946) shown in Fig 8B. Based on 3σ/s (σ is the standard deviation of the blank measurements and s is the sensitivity of the calibration graph), the detection limit of sarcosine was calculated to be 0.73 μM. Such performance characteristics suggested a potential use of as-proposed colorimetric platform for sarcosine sensing.
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In summary, a simple and rapid colorimetric nanosensor with high selectivity and sensitivity based on NQS functionalized GO nanocomposite through π-π stacking was explored. The
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