Simple and Cost Effective Polymer Modified Gold Nanoparticles Based on Colorimetric Determination of L-Cysteine in Food Samples

Beeta Rani Khalkho, Anushree Saha, Bhuneshwari Sahu, Manas Kanti Deb*

School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur-492010, Chhattisgarh, India

*Corresponding author: debmanas@yahoo.com

[Received: 18 February 2021; Accepted: 25 May 2021]

Abstract. The purpose of the present research was to design a method for the colorimetric determination of L-cysteine. We have employed PVA capped gold nanoparticles (GNPs) as a probe. The as-synthesized GNPs were further characterized by UV-vis absorption spectroscopy, transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and Zeta potential analyser. The results show that the presence of L-cysteine caused the quenching of the surface plasmon resonance band of the GNPs at 524 nm. It was accompanied by the appearance of a new absorbance of a new absorbance band at 670 nm. The color of the colloidal GNPs changed from wine red to blue. The change in color of the GNPs was due to their aggregation induced by the presence of L-cysteine. Based on these observations, the as-synthesized GNPs were utilized to develop a novel colorimetric sensor for L-cysteine detection in food samples. Significantly, other biomolecules such as alanine, proline, phenylalanine, tryptophane, valine, arginine, glutamic acid, lysine and histidine did not cause any change in the color of the GNPs solutions. This colorimetric probe showed excellent selectivity and high sensitivity for L-cysteine with a detection limit of 2.0 μg mL^-1.

Keywords: PVA capped gold nanoparticles; electrostatic interaction; aggregation; colorimetric probe; L-cysteine; food samples.

Introduction

L-cysteine is sulfur containing non-essential amino acid and can be found as a component of many proteins throughout the body [Hajizadeh et al., 2012]. L-cysteine has a critical role in a variety of main cellular functions, such as metabolism, protein folding, detoxification, redox and bio catalysis, so that its deficiency causes many syndromes, such as hair depigmentation, liver damage, skin lesions and muscle contraction [Bamdad et al., 2016; Chen et al., 2014]. Thus the development of suitable methods for the detection of L-cysteine in various samples has been of considerable interest in recent years. Several methods have been reported for the determination of L-cysteine, such as high-performance liquid chromatography (HPLC) [Amarnath et al., 2003] gas chromatography with mass spectrometry (GC–MS) [Santhoshkumar et al., 1994], spectrofluorimetry [Anand et al., 2014] and chemiluminescence [Chaichi et al., 2014]. These methods have major disadvantages such as poor sensitivity, high detection limit, require large amount of organic reagent, time consuming and depend on high experienced operators for the sample preparation. The spectrophotometric method is simple, rapid and provides a lower cost, which is available in small laboratories as well as for the on-site analysis of food samples. Thus, in the present work, colorimetric method for determination of L-cysteine in food samples is investigated.

Metal nanomaterials have attracted much attention due to their remarkable chemical, physical, visual, electronic, magnetic, and catalytic properties [Nidya et al., 2014; Cui et al., 2015]. The synthesis of metal nanomaterials, nanoclusters in particular, has a continuing need for simple and effective methods. Ligand covered metal NPs increase the surface-to-volume ratio and have been increasingly studied as analytical devices in various fields [Smitha et al., 2008; Khan et al., 2013]. The incorporation of organic ligands onto NPs surfaces provides not only stability to these nano entities, but also important surface functionalities which make them good candidates for specific applications [Li et al., 2010]. Among the nanomaterials used as constituent in sensing, GNPs have received greatest interests because of their unique properties, such as surface plasmon resonance (SPR) absorptions in the visible region, conductivity, simple preparation, high stability, versatile surface chemistry and good biocompatibility [Denga et al., 2014]. The accumulation of GNPs contributes in theory to interparticle plasmon couplings, and the red change and enlargement of the plasmon band is especially beneficial. The changing of color during aggregation of silver nanoparticles leads to a widespread application for the absorption-based colorimetric sensing of any target that allows gold nanoparticles to accumulate directly or indirectly. Many analysis methods were developed based on GNP, including colorimetry, electrochemistry, fluorescence, quartz crystal microbalance improved surface dispersal and bio-barcode testing. [Saha et al., 2012]. Among these approaches, colorimetry, mostly based on the SPR properties of GNPs, has attracted fast-growing interest because the analyte recognition events can be easily monitored by the naked eye with high sensitivity, simplicity, rapidity, minimal material consumption, no requirement of any advanced instruments and is cost effective. In such assays, a transition in the color of the accumulation of GNPs is caused in the presence of an analyte of interest. A number of GNP-based colorimetric methods for sensing different substances, including biomolecules, ions and small organic molecules, have been developed [Zhang et al., 2012; Haghnazari et al., 2013; Jiang et al., 2010].  We have used PVA capped GNPs for the determination of L-cysteine as an SPR-based assay in present work.

Experimental design

Materials

All chemicals and reagents used were in analytical grade in the present study. Chloroauric acid (HAuCl₄), and polyvinyl alcohol (PVA) were purchased from Sigma-Aldrich (ACS reagent, ≥99%, MA, USA). Infrared spectroscopic grade potassium bromide (KBr) was purchased from Merck (KGaA Darmstadt, Germany). Tris(2-carboxyethyl) phosphine hydrochloride (TCEP), m-phosphoric acid (MPA),  sodium hydroxide (NaOH), hydrochloric acid (HCl), L-cysteine, alanine, proline, phenylalanine, tryptophane, valine, arginine, glutamic acid, lysine and histidine were obtained from Himedia (India). Ultrapure water (18.2 M to cm) was used to prepare all the solutions for this analysis.

Synthesis of the PVA capped GNPs

All glassware was carefully washed with freshly formulated solution 3:1 HCl: HNO3 (aqua regia) and rinsed in ultra-pure water and then dried in a dry oven prior to use. GNPs have been prepared according to the Frens system with certain variations [Panigrahi et al., 2007]. Briefly, 50 mL of 0.002 M HAuCl₄ solution was heated to boiling and 1 mL of 1% PVA solution was added to it under continuous stirring. The solution turned faint blue within 20 s of boiling. Under continuous stirring condition, After 70 s the blue color suddenly changed into wine red signifying the formation of GNPs. The reaction mixture solution was boiled for ~30 min for completion of the reaction. The synthesized PVA capped GNPs solutions was stored at room temperature. Absorption measurement of the finally formed gold colloid obtained an intense absorption band with a maximum at 524 nm. PVA capped GNPs were synthesized according to a very simple and rapid Frens method as follows in Fig. 1.

Samples collection and preparation for determination of L-cysteine using PVA capped GNPs

For the analysis of Fruit (Lemon, Papaya and Mango) were collected from different sites of Chhattisgarh, India and their juices were extracted. The samples were diluted (final volume 50 mL) with a concentrated extraction-reduction solution (5 % MPA, 2.5 mM TCEP, and 2.5 mM EDTA) and heated at 40 °C for 60 min in a forced air oven and then cooling. Then filtered using Whatman filter paper (0.45μm pore size) for analysis.

General procedure for the UV-vis spectral measurements

For the detection of L-cysteine, different concentrations of L-cysteine were added into a glass vial containing 5 mL of PVA capped GNPs solution and stirred to mix these uniformly. Then the solution mixture was kept for 4 min of reaction time. During this time the color of the solution completely turned from wine red to blue. The solution mixture was then transferred to quartz cuvettes to monitor the absorption spectra in the range of 200-800 nm. Different known concentrations of L-cysteine were applied to a solution containing 1 mL of GNP and 1 mL of diluted samples, and the pH of the sample solution was held at 5.0 using 0.1 M HCl and 0.1 M NaOH solutions after the absorption spectra was measured. Both absorption tests have been conducted in triplicate.

Apparatus

The UV-Vis spectra of GNPs were recorded by a Cary 60 (Agilent Technologies) UV-Vis spectrophotometer, using quartz cuvettes of 1.0 cm path length. The absorption spectra were recorded in the UV-vis range from 200-800 nm for the determination of L-cysteine using PVA capped GNPs in food samples. IR spectra were measured on a Nicolet is10 FTIR spectrometer (Thermo Fisher, Scientific Madison, USA). The size and their distribution of GNPs and aggregated GNPs with L-cysteine were recorded by dynamic light scattering (DLS) Zetasizer Nano ZS 90 (Malvern Instrument, Japan). The transmission electron microscope (TEM) from Jeol (IET, 2200 FS) was used to determine the size and shape of disperse and aggregated PVA capped GNPs with L-cysteine. Thermo Fisher Scientific Barnstead Smart2pure water system (Conductivity 18.2 Ω^-1) was used to obtain ultrapure water for solution preparations. A Systronics optical pH meter (type-335) was used to calculate the pH of the solution.

Results and discussion

Characterization of GNPs

UV-Vis spectrophotometric technique is used for characterization and quantification of mixture solution on the basis of SPR band through the color change. The absorption spectra of the GNPs were obtained a single peak at a wavelength of 524 nm, which is due to SPR phenomenon. A strong band at 524 nm showed the formation of stable GNPs, and the addition of L-cysteine into the NPs caused the aggregation of particles resulting in a change of solution color from wine red to blue. The addition of L-cysteine into the GNPs caused the SPR band to shift to a higher wavelength from 520 nm to 670 nm, as shown in Fig. 2(a-b). The reason for the red shift of the SPR band was the aggregation of the GNPs. The color of the resulting suspension of the GNPs clearly changed from wine red to blue (inset of Fig. 2), indicating the aggregation of the GNPs. The dispersed GNPs and aggregated GNPs with L-cysteine were used to analyse the particle size (hydrodynamic diameter) by DLS and the zeta potential analyser was performed to determine the potential stability in the colloidal suspension. The average hydrodynamic diameter of GNPs was 56.2 ± 5.8 nm confirming the excellent dispersion property of GNPs in aqueous solution (Fig. 3a). However, the average hydrodynamic diameter of GNPs resulted in five-fold enhancement in the size of NPs (324.0 ± 13.4 nm) in the presence of L-cysteine caused by the aggregation induced by the addition of analytes as shown in Fig. 3b. The size and particle morphologies of the GNPs before and after interaction were studies using TEM analysis. As shown in Fig. 3(c) shows the particles were found to be mostly spherical in shaped with average particle size of 9.0 nm. Well dispersed without the formation of aggregated NPs. As shown in Fig. 3(d) shows indication towards morphological variations and aggregation of GNPs upon addition of L-cysteine with average particle size greater than 10 nm. These findings clearly indicated that the incorporation of L-cysteine into the GNPs contributed to the accumulation of NPs. In order to investigate the GNP before and after the addition of L-cysteine FTIR spectra, KBr disks of powdered samples were analyzed with a resolution of between 400 and 4000 cm-1 and 4 cm-1.

Mechanism for the determination of L-cysteine using the GNPs as a chemical probe

In this work, the colorimetric method is used for the selective determination of L-cysteine using GNPs in food samples. The different amino acid such as alanine, proline, phenylalanine, tryptophane, valine, arginine, glutamic acid, lysine and histidine were tested for the selective detection of amino acid using PVA capped GNPs as a colorimetric probe. For this, all the amino acid and NPs were separately taken in 5 mL glass bottle in the volume ratio of 1:1 while maintaining the pH of sample to 5.0 and the solution mixture were incubated for 4 min at room temperature until instant coloration. The NPs solution with amino acid such as alanine, proline, phenylalanine, tryptophane, valine, arginine, glutamic acid, lysine and histidine showed a SPR absorption peak at 524 nm, which were found similar to UV-Vis spectrum of disperse GNPs as shown in Fig. 2 (a). However, on addition of L-cysteine into the NPs solution of, the plasmon band at 524 nm was shifted along with appearance of new and strong absorption peak at about 670 nm, owing to the absorbance of the aggregated GNPs as shown in  Fig. 2 (b).  Meanwhile, the color of NPs solution was changed from wine red to blue, while the NPs solutions containing other amino acid did not show any color change, demonstrating that there was no surface interaction with the NPs. Thus, the change in solution color from wine red to blue and appearance of a new SPR absorption band at 670 nm for L-cysteine with NPs were used as a colorimetric assay method for selective determination of L-cysteine in food samples. Our results were significantly in accordance with the already published evidence on silver nanoparticles (SNPs) and L-cysteine [Ravindran et al., 2011]. Interaction of GNPs with L-cysteine based on the study of UV-vis absorption spectra, the authors support the presence of covalent interactions between S and Au. The formation of a covalent bond Au-S is often presumed by other scholars [Shrivas et al., 2017].  The color of GNPs was changed from wine red to blue in the presence of L-cysteine that indicated the aggregation of NPs with analytes causing effective absorption of UV light. This bond is formed by ligand exchange reactions between L-cysteine and PVA capped GNPs.

      Further, we verified the size, shape and morphologies of GNPs and aggregated NPs with and without L-cysteine in aqueous solution by performing TEM measurements (Fig. 3 c-d). We observed from Fig. 3 (c) that in the absence of L-cysteine, GNPs are roughly sphere-shaped with an average size of 9.0 nm Fig. 3 (d) shows indication for the morphological variations and aggregation of NPs depending upon the addition of L-cysteine and the average particle size ˃10 nm was observed. We also confirmed the size and percentage distribution of GNPs in presence and absence of L-cysteine using DLS measurements and results are shown in Fig. 3(a-b).  The hydrodynamic diameter of the prepared GNPs was found to be 56.2 ± 5.8 nm and after the addition of L-cysteine the hydrodynamic diameter of 324.0 ± 13.4 nm. Before the addition of L-cysteine, PVA capped GNPs have a zeta potential of -12 mV, this high value refers to high stability and excellent capping of PVA on the surface of GNPs. Upon addition of L-cysteine, zeta potential was 24 mV implying the reduced stability of GNPs. The results are obtained in Fig. 4 (a-b).  There for DLS and the zeta potential observations were also in substantial agreement with the spectroscopic data and the microscopic observation confirming the interaction.

     The polymer functions as a binder and also it prevents the process of agglomeration of GNPs and finally limits the diameter of NPs formed. The electrostatic interactions between PVA and GNPs were studied by FTIR spectroscopy. Fig. 5 (a) shows the FTIR spectra of pure PVA, PVA modified GNPs, pure L-cysteine and PVA modified GNPs with L-cysteine. The PVA spectrum exhibits a very strong broad band at 3320.77 cm^-1 which is assigned to O-H stretching vibration indicating the presence of hydroxyl groups.  The absorption bands obtained at 2940.64 and 1716.53 cm^-1 correspond to C–H and C=O stretching vibrations, respectively and the absorption band at 1579.61 cm^-1 arises due to the C=C stretching. The absorption bands at 1430.46 cm-1 is assigned to CH2 bending and that at 1374.15 cm^-1 is due to CH₂ wagging [Mahendia et al., 2013]. The bands appeared at 1260.77 cm^-1 and 1089.80 cm^-1 is due to C-H wagging and C-O stretching respectively. The shift of band towards the higher wave number exhibited the modification of GNPs with PVA molecules. Next, the IR spectra of pure L-cysteine and the solution mixture of the GNPs with L-cysteine were recorded. The results are shown in Fig. 5(b). The weak band attributed to -SH group of L-cysteine molecule is observed at 2552.35 cm^-1. The bands obtained at 1625.00 and 1395.66 cm^-1 corresponded to the asymmetric and symmetric stretches of carboxylate group (COO^- ). An exhibited band at 3320.40 cm^-1 signified amine group (NH₃⁺) stretches and its bending vibration at 1519.10 cm^-1. The decrease in the signal intensity and the shift of the peaks were observed for the solution mixture of GNPs with L-cysteine compared to pure L-cysteine molecules, representing the interaction of L-cysteine with the NPs. Fig. 5(b) illustrates nearly similar vibrational bands. However, the band belonging to -SH (2552.35) is not observed in this spectrum which confirms the strong covalent Au–S bonding [Chai et al., 2010; Soomro  et al., 2014; Xinfu et al., 2016]. The stretching vibration of NH₃⁺ shifted from 3320.40 to 3360.70 cm^-1 and C-H shifted from 2940.64 to 2988.26  cm^-1. In addition, on the basis of changes in dipole, minor changes in frequencies can be defined as an importance for the binding of L-cysteine on a high-level metal surface for the spectrums of PVA-modified GNPs with L-cysteine.

Based on the above experimental results, we proposed a possible model illustrating both the L-cysteine bindings to GNPs and the formation of particle aggregates is schematized in Fig. 6 (a) and (b). Here, PVA molecules was used as a reducing agent to convert Au³⁺ to Au⁰ as well as a capping agent to stabilize the surface of the NPs and stabilize them for a long time. The prepared PVA capped GNPs showed a wine red color in aqueous solution, which could be used as a colorimetric probe for the detection of L-cysteine. After L-cysteine adsorption on the GNPs, the L-cysteine molecule has still two functional groups free to form bonds between particles. The positive NH₃⁺ group of L-cysteine is capable to form salt bridges with COO^- group which result in the aggregation of the GNPs. However, the color of the GNPs changed from wine red to blue due to the agglomeration of NPs caused due to the removal of PVA molecules from surface of NPs, shown in Fig. 6 (b).  This is due to the high affinity thiol present in the L-cysteine because they can bind electron rich sites strongly. Besides the electrostatic interaction mechanism by salt bridges, a possible binding of thiol group on the surface of the GNPs through covalent bonding has also mentioned [Mocanua et al., 2009]. Therefore, the aggregation of NPs results in the red shift and appearance of SPR absorption in the UV-Vis spectra, which accounts for the colorimetric assay for the determination of L-cysteine in food samples.

The sensitivity of the GNPs probe towards L-cysteine was ascertained by the changes in the SPR band of PVA capped GNPs with different concentration of L-cysteine solutions ranging from 50-500 μg mL^-1 and the corresponding results are displayed in Fig. 7. It can see from the SPR spectra, the presence of L-cysteine led to the red-shift of the peak at 524 nm and a new peak appeared at 670 nm. Meanwhile, the color of GNPs was changed from wine red to blue. The increase of L-cysteine concentration would induce a decrease in the absorption intensity of GNPs at 524 nm accompanying an obvious enhancement of new peak 670 nm. The significant red-shift and decreasing absorption intensity is due to the aggregation of GNPs and L-cysteine by the electrostatic interaction, which was confirmed the aggregation by the TEM characterization. This phenomena attributed to the fact that, the positively charged amino group in L-cysteine (-NH³⁺) should interact with the negative charged carboxylate group (COO-) on the surface of other GNPs through electrostatic binding, thus forming assemblies.  Therefore, the results indicated that the absorption ratio at 524/670 nm was taken for the quantitative analysis of L-cysteine in food samples.

Optimization of the proposed method

The influence of the assay conditions including media pH, stirring rate, reaction time and concentration of NPs was investigated. The efficient detection of L-cysteine was obtained when pH sample solution was 5.0 at stirring rate of 300 rpm for 4 min of reaction time using 4.0 μM concentrations of GNPs. The results are given in Fig. 8 (a-d).

Effect of pH

The effect of pH on the interaction between GNPs and L-cysteine was studied. The pH was performed by adjusting pH of the L-cysteine solution at pH 2, 3, 4, 5, 6, 7, 8 and 9 by adding HCl (0.1 M) or NaOH (0.1 M) solution. Fig. 8a shows the absorbance ratio (A_670/524nm) increased in the pH range of 3.0-5.0, while lower or higher pH would slow down the aggregation kinetics of the particles. Note that the zwitterionic isoelectric point of L-cysteine was 5.0, facilitating the aggregation of the particles [Chen et al., 2014]. Thus, pH 5.0 was selected for further experiments.

Effect of reaction time

The effect of incubation time on the aggregation of GNPs induced by L-cysteine was investigated and optimized. The incubation time is also a key factor, affecting colorimetric results. Fig. 8b showed that the increment of the absorbance ratio (A₅₂₄/A_670nm) first increased with an increase in the reaction time (0-4 min); beyond decrease absorbance. Based on these results, the as mixed solution was incubated for 4 min for the following detection experiment.

Effect of GNPs concentration

The  effect of PVA capped GNPs concentration on the colorimetric determination of L-cysteine was optimized. Fig. 8c. showed the evolution of SPR UV-Vis spectra upon the adding 50 μg mL^-1 of L-cysteine to different concentration of GNPs (1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 μM). It was found that the increasing GNPs concentration caused increase the absorption intensity. However, at concentration level of 4.0 μM of GNPs, no significant increase in absorption intensity was observed. Therefore, 4.0 μM of GNPs was chosen as optimum concentration of PVA capped GNPs.

Effect of stirring rate

The effect of stirring rate were also studied for determination of L-cysteine for chemical reaction between L-cysteine and GNPs. Stirring rate from 100 to 350 rpm was applied for aggregation process using 4 min at pH 5.0. The results are displayed in Fig. 8d. The absorbance ratio (A₅₂₄/A_670nm)  in UV-Vis spectrophotometer of the analyte was increased with increasing stirring rate up to 300 rpm, and remained steady at 300 rpm. Therefore, a stirring rate of 300 rpm was used for further experiments.

Effect of potentially interfering compounds

The effect of several potentially interfering compounds including amino acids (alanine, proline, phenylalanine, tryptophane, valine, arginine, glutamic acid, lysine and histidine), metallic ions (Li⁺, Na⁺, Ca²⁺, Ba²⁺, Ni²⁺, Cu²⁺, Co²⁺) carbohydrates (D-fructose, sucrose) that may be present in food samples is evaluated for the selective and specificity determination of L-cysteine with and without using GNPs. The color as well as spectral changes of GNPs upon the addition of various interfering metal ions is presented in Fig. 9. The UV-Vis absorption band of GNPs remained unchanged in the presence of other biomolecules and metal ions at the optimized conditions, while only L-cysteine displayed the color change from wine red to blue as well as red shift of SPR absorption band. Selectivity defines whether analytical method could differentiate interference of similar group of compounds in the presence of analytes which is determined by relative absorbance value by colorimetric method. The ratio of absorbance intensities at A₅₂₄/A₆₇₀ nm was used to assess the degree of GNPs aggregation in colorimetric measurements. Therefore, the colorimetric method was found free from interference of diverse substances and other biomolecules which are commonly related with the determination of L-cysteine.

Analytical evaluation for determination of L-cysteine using GNPs

In order evaluate the performance of GNPs for detection of L-cysteine, important parameters such as linear range, limit of detection (LOD), limit of quantification (LOQ), accuracy, precision, correlation coefficient (R²) and selectivity were examined. Quantitative analysis was performed by the adding various concentrations of L-cysteine into 4.0 μM GNPs solution and monitoring the absorption peak of the mixing solution by UV-Vis spectra and color changes after 4 min. A calibration curve was drawn by plotting the absorbance ratio (A₅₂₄/A_670m ) against concentration of L-cysteine. The calibration curve was linear over the concentration of L-cysteine ranging from 50-500 μg mL^-1 (50, 100, 150, 200, 250, 300,400 and 500 μg mL^-1). A good linearity was obtained in the range of 50-500 μg mL^-1 of L-cysteine with a correlation coefficient (R²) of 0.994, as shown in Fig.10 (a-b). The LOD and LOQ for L-cysteine detection were found to be 2.0 μg mL^-1and 6.2 μg mL^-1 respectively. The LOD of the proposed method is superior to the other methods. As shown in Table 1. It is clearly seen that the developed GNPs sensor shows good sensitivity and detection range with the acceptable for common detection. To examine the precision of the method was obtained by calculating the relative standard deviation percentage (RSD %) by three replicate analysis of the samples under the optimized conditions. The RSD for determination of L-cysteine was found to be 1.8 % showing a good precision of the method for determination of L-cysteine in sample solution. The results, given in Table 1 and compared with results obtained Fourier transform infrared spectroscopy [Ahmadi et al., 2007].

Application for determination of L-cysteine in food samples using GNPs

In order to evaluate the applicability and reliability of the our proposed colorimetric sensor, PVA capped GNPs was applied for determination of L-cysteine in food samples.  An aliquot of filtered samples (1.0 mL) was added into a glass bottle containing 1.0 mL of GNPs at pH 5.0. The solution mixture was kept at 4 min incubation time while maintaining the pH of sample solution. SPR absorption peak obtained at 670 nm in colorimetric analysis was used for determination of L-cysteine from food samples. In order to determine the applicability and consistency of the proposed method, three food samples including lemon, papaya and mango were selected for the determination of L-cysteine. In this present work, the quality control experiment was carried out for each samples consisting of a linear calibration standard in matrix, a blank and a spiked samples for the compounds. Total 6 numbers of food samples were tested those were obtained from Raipur city, India. Out of all samples, three types of food samples were found positive towards the presence of L-cysteine in significant concentrations. The results on the L-cysteine contents of all positive samples are shown in Table 2. The recovery % was calculated by spiking two different concentration of L-cysteine to the actual sample. As can be seen from Table 2, the recoveries can be quantitative in the range of 97.7-103.3 % with the RSD less than 2.3 %, indicating the potential applicability of our method in food samples.

Next, Table 3 demonstrated the determination of L-cysteine using GNPs as a chemical probe in colorimetric was compared with the results of other reported methods in terms of linearity range, LOD, RSD and recovery %. Lower LOD values were obtained with GNPs as a sensing probe in colorimetric compared with fluorimetry, Electrochemiluminescence and cyclic voltammetry etc. [Agui et al., 2007; Chen et al., 2020; Abbas et al., 2015; Kataoka et al., 1995]. These previous described methods are the use of large amounts of substances and time-consuming complex sample preparation steps before analysis, needs skilled personnel and high cost chemical reagents. The present method based on PVA capped GNPs chemical sensor is very simple, sensitive, selective, and cost effective and also small amount of chemical reagents are used with other sophisticated analyticalmethods.