of Ravishankar University–B, 34 (1), 41-57 (2021)
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
School of Studies in Chemistry, Pt. Ravishankar Shukla University,
Raipur-492010, Chhattisgarh, India
*Corresponding author: email@example.com
[Received: 18 February 2021; Accepted: 25
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.
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.
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.
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
Synthesis of the
PVA capped GNPs
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.
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.
procedure for the UV-vis spectral measurements
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.
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.
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.
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.
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.
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.
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 (A670/524nm) 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.
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/A670nm) 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
Effect of GNPs
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.
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/A670nm) 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
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.
evaluation for determination of L-cysteine using GNPs
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/A670m ) 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).
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].
Table 1. Determination of L-cysteine food samples analysed by the present method
and reported Fluorescence
data for L-cysteine analysis
Linear range (µg mL−1)
Correlation estimation (R)
Correlation Coefficient (R2)
Concentration range (µg mL−1)
LOD (µg mL−1)
LOQ (µg mL−1)
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.
Table 2. Recovery
percentage (%) for determination of L-cysteine using polymer capped AuNPs in food samples
(experimental conditions: = 1 mL of AuNPs; stirring rate = 300 rpm; reaction
time = 4 min; pH = 5.0)
14.6 ± 0.34
21.8 ± 0.28
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
Comparison of the present method with other reported methods for
determination of L-cysteine
5 - 50 μM
13 - 350 μM
PVA capped AuNPs
50-500 µg mL-1
2.0 µg mL-1
nitride/glassy carbon electrode, bCadmium tellurium quatum dots, cCopper
The PVA capped
GNPs have been demonstrated as a colorimetric probe for the selective
determination of L-cysteine. The change in color of the colloidal GNPs from
wine red to blue was due to the aggregation of the particles in the presence of
L-cysteine. The aggregation of NPs induced by L-cysteine due to strong electrostatic
interaction between the positively charged NH3+ of the
first molecule with the negatively charged COO- ions of the second
molecule present on the surface of GNPs. The colorimetric method was applied for
the qualitative determination of L-cysteine in food samples.
Khalkho is thankful to Council of Scientific & Industrial Research (CSIR),
New Delhi for the financial support. The authors are also grateful for
Financial assistance from DST-FIST [No.-SR/FST/CSI-259/2014(c)]. The authors
are thankful to the University Grants Commission Special Assistance Programme
[no. F-540/7/DRS-II/ 2016 (SAP-I)] for financial support. The authors are
thankful to Prof. Shamsh Pervez, Head, School of Studies in Chemistry, Pt.
Ravishankar Shukla University, Raipur for providing laboratory facilities.
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