A. Rajeshwari

Simple colorimetric detection of Cr(III) in aqueous solutions by as synthesized citrate capped gold nanoparticles and development of a paper based assay

In the current study, we present a colorimetric sensing method for Cr(III) in aqueous solution using synthesized gold nanoparticles (AuNPs) without any surface functionalization. The method is based on the aggregation of citrate capped AuNPs by Cr(III) leading to a red-shift of the UV-visible absorption maxima from 526 nm to 714 nm. Dynamic light scattering measurements and scanning electron microscopy confirmed the aggregation of the AuNPs. Under the optimized experimental conditions, a linear relationship (correlation coefficient r = 0.997) was established between the ratio of the absorbance at 714 nm to that at 526 nm (A714/526) and the concentrations of Cr(III) over a range of 10−3 to 10−6 M [50 000–50 μg L−1]. The reported probe had a limit of detection of 1.06 × 10−7 M [5.3 μg L−1] and demonstrated good selectivity towards Cr(III). The applicability of the technique was extended to a simple paper based assay, wherein the AuNPs immobilized on Whatman filter paper strips interacted with Cr(III) and the color intensity measured was calibrated against the analyte concentrations. Under the optimized conditions, a linear relationship (correlation coefficient r = 0.99) was noted between the measured color intensity and the concentration of Cr(III) within the range of concentrations 10−3 to 10−6 M and the theoretical limit of detection was computed to be 1.53 × 10−7 M. The paper strip based method possessed high selectivity towards Cr(III), and was also successfully tested against an artificial mixture of interfering metal ions. The current study is the first of its kind to report the development of a paper strip based assay for the colorimetric sensing of chromium using as synthesized AuNPs without any surface functionalization, and may facilitate the on-site monitoring of Cr(III) in contaminated sites.

Toxicity evaluation of gold nanoparticles using an Allium cepa bioassay

The progressive increase in the usage of gold nanoparticles (NPs) in industrial and commercial products leads to the potential release of nanoparticles into the environment, which could cause adverse effects on living systems. In the present work, the size- and dose-dependent cytogenetic effects of gold NPs towards a plant system were evaluated for the first time by a simple and cost-effective Allium cepa (A. cepa) bioassay. Citrate-capped gold NPs of three different sizes, 15 (Au15), 30 (Au30), and 40 (Au40) nm, were synthesized by a citrate reduction method. The mean hydrodynamic diameter and morphology of as-synthesized gold NPs were characterized by dynamic light scattering and transmission electron microscopy analyses. Several chromosomal aberrations were observed under an optical microscope upon the exposure of A. cepa root tip cells with 0.1, 1, and 10 μg mL−1 of Au15, Au30, and Au40. The mitotic indices in treated root tips were directly proportional to the NP concentration and inversely related to their size. The vehicle control (citrate) had no impact on the mitotic index. Furthermore, the effects of gold NPs on the A. cepa root tip were confirmed by analyzing the generation of various reactive oxidant species (hydroxyl, superoxide and hydrogen peroxide), which possibly led to lipid peroxidation in the system.

Colorimetric detection of melamine based on the size effect of AuNPs

A simple colorimetric detection of melamine was studied using 15 nm (AuNPs-I), 30 nm (AuNPs-II), and 40 nm (AuNPs-III) citrate-capped gold nanoparticles (AuNPs). The AuNPs aggregated in aqueous solution in the presence of melamine, showing a visual color change from red to blue. This color change led to a shift in the absorption peak from 527 nm, 526 nm, and 525 nm to 638 nm, 626 nm, and 680 nm for AuNPs-I, AuNPs-II, and AuNPs-III, respectively. For all the three AuNPs, linearity was observed between the melamine concentration in aqueous solution and the absorbance ratios, A638/527, A626/525, and A680/526, respectively. The limit of detection (LOD) for melamine for the AuNPs-II was found to be 2.37 × 10−8 M (correlation coefficient R2 = 0.9745), which showed better sensitivity as compared to the LOD of the AuNPs-I and AuNPs-III, which were 3.3 × 10−8 M and 8.9 × 10−8 M, respectively. The synthesis of AuNPs-II also involved a lower HAuCl4 concentration compared with the other two types of AuNPs, which may reduce the process cost. The AuNPs-II was selected to analyze melamine in pre-treated milk samples, and the recovery percentage was in the range of 91–106%. Thus, the efficient detection of melamine was possible using AuNPs-II for the on-site detection without the aid of expensive instruments.

Simple colorimetric sensor for Cr(III) and Cr(VI) speciation using silver nanoparticles as a probe

In this study, we present a simple colorimetric probe for detecting chromium species, i.e. Cr(VI) and Cr(III), in aqueous solution using as-synthesized citrate-capped silver nanoparticles without further functionalization. Two types of silver nanoparticles, AgNP-I (10 ml sodium borohydride) and AgNP-II (30 ml sodium borohydride) were synthesized by varying the volume of the reducing agent. AgNP-I was proven to be specific for Cr(III) only whereas AgNP-II would measure total chromium in a binary mixture of Cr(III) and Cr(VI). The probe was tested in a binary mixture at a 1 : 1 ratio, containing the total chromium concentrations in the range of 500–5000 ppb. The sensing method was also successfully applied in a portable colorimeter with similar binary combinations paving the way for further on-site applications.

Studies on fluorescence determination of nanomolar Cr(III) in aqueous solutions using unmodified silver nanoparticles

In the current study we present a fluorescence probe for Cr(III) in aqueous solution employing as-synthesized silver nanoparticles (Ag NPs) without surface labeling by any extrinsic fluorophore. The principle of the method involves aggregation of Ag NPs by Cr(III) leading to a red-shift of the fluorescence emission peak from 420 to 684 nm for a corresponding excitation wavelength of 340 nm. The UV-visible absorption spectral studies, dynamic light scattering measurements, and scanning electron microscopy (SEM) corroborated the aggregation of the Ag NPs upon interaction with Cr(III). Under the optimized conditions, a good linear relationship (correlation coefficient r = 0.975) was observed between concentration of Cr(III) and the fluorescence intensity at 684 nm in the range of 1 × 10−3 to 2 × 10−9 M (50 mg L−1 to 100 ng L−1). The reported probe has a limit of detection as low as 2 nM, which, to the best of our knowledge, is the lowest ever reported for the fluorescence detection of Cr(III). The developed sensor was successfully tested with chromium containing real samples, such as groundwater and fresh water, demonstrating potential application in the field of environmental sensing.

Developing acetylcholinesterase-based inhibition assay by modulated synthesis of silver nanoparticles: applications for sensing of organophosphorus pesticides

A novel and highly sensitive sensing strategy for the detection of organophosphorus compounds (OPs) based on the catalytic reaction of acetylcholinesterase (AChE) and acetylcholine (ATCh) during the modulated synthesis of silver nanoparticles (AgNPs) has been developed. The enzymatic hydrolysis of ATCh by AChE yields thiocholine (TCh), which induces the aggregation of AgNPs during synthesis, and the absorption peak at 382 nm corresponding to AgNPs decreases. The enzymatic reaction can be regulated by OPs, which can covalently bind to the active site of AChE and decrease the TCh formation, thereby decreasing the aggregation and significantly enhancing the absorption peak at 382 nm. The proposed system achieved good linearity and limits of detection of 0.078 nM and 2.402 nM for trichlorfon and malathion, respectively, by UV-visible spectroscopy. Further, the sensitivity of the proposed system was demonstrated through the determination of OPs in different spiked real samples. The described work shows the potential application for further development of a colorimetric sensor for other OP pesticide detection during the synthesis of AgNPs using enzyme-based assays.

Cytogenetic studies of chromium（III） oxide nanoparticles on Allium cepa root tip cells

The current study evaluates the cytogenetic effects of chromium (III) oxide nanoparticles on the root cells of Allium cepa. The root tip cells of A. cepa were treated with the aqueous dispersions of Cr2O3 nanoparticles (NPs) at five different concentrations (0.01, 0.1, 1, 10, and 100μg/mL) for 4hr. The colloidal stability of the nanoparticle suspensions during the exposure period were ascertained by particle size analyses. After 4hr exposure to Cr2O3 NPs, a significant decrease in mitotic index (MI) from 35.56% (Control) to 35.26% (0.01μg/mL), 34.64% (0.1μg/mL), 32.73% (1μg/mL), 29.6% (10μg/mL) and 20.92% (100μg/mL) was noted. The optical, fluorescence and confocal laser scanning microscopic analyses demonstrated specific chromosomal aberrations such as-chromosome stickiness, chromosome breaks, laggard chromosome, clumped chromosome, multipolar phases, nuclear notch, and nuclear bud at different exposure concentrations. The concentration-dependent internalization/bio-uptake of Cr2O3 NPs may have contributed to the enhanced production of anti oxidant enzyme, superoxide dismutase to counteract the oxidative stress, which in turn resulted in observed chromosomal aberrations and cytogenetic effects. These results suggest that A. cepa root tip assay can be successfully applied for evaluating environmental risk of Cr2O3 NPs over a wide range of concentrations.

Determination of mercury(II) ions in aqueous solution using silver nanorods as a probe

We report here the selective and sensitive determination of Hg2+ in aqueous solution using the aggregation and re-aggregation of silver nanorods (SNRs) in the presence of dithiothreitol (DTT). The SNRs covalently bound with the thiol groups at the ends of the DTT molecule, leading to the aggregation of the SNRs. However, the aggregation of the SNRs was prevented by the addition of Hg2+. The thiol group of DTT has a strong binding affinity with Hg2+ that helps to prevent the aggregation of SNRs. UV-visible spectrophotometry and transmission electron microscopy confirmed the aggregation and re-aggregation of SNRs with DTT and Hg2+. The absorption ratio (A/A0) of the SNRs at λ585 nm decreased after reaction with DTT and increased with the addition of Hg2+. The changes in the absorption ratio of the SNRs at λ585 was directly proportional to the concentration of Hg2+ in the range 1–100 pM. The developed method had good linearity (y = 0.001x + 0.899; R2 = 0.968) under the optimized conditions with a limit of detection of 0.15 pM. The method was validated in real samples such as tap and lake water and could be used for the environmental sensing of Hg2+.

Anti-aggregation-based spectrometric detection of Hg(II) at physiological pH using gold nanorods

An efficient detection method for Hg (II) ions at physiological pH (pH7.4) was developed using tween 20-modified gold nanorods (NRs) in the presence of dithiothreitol (DTT). Thiol groups (-SH) at the end of DTT have a higher affinity towards gold atoms, and they can covalently interact with gold NRs and leads to their aggregation. The addition of Hg(II) ions prevents the aggregation of gold NRs due to the covalent bond formation between the -SH group of DTT and Hg(II) ions in the buffer system. The changes in the longitudinal surface plasmon resonance peak of gold NRs were characterized using a UV-visible spectrophotometer. The absorption intensity peak of gold NRs at 679nm was observed to reduce after interaction with DTT, and the absorption intensity was noted to increase by increasing the concentration of Hg(II) ions. The TEM analysis confirms the morphological changes of gold NRs before and after addition of Hg(II) ions in the presence of DTT. Further, the aggregation and disaggregation of gold NRs were confirmed by particle size and zeta potential analysis. The developed method shows an excellent linearity (y=0.001x+0.794) for the graph plotted between the absorption ratio and Hg(II) concentration (1 to 100pM) under the optimized conditions. The limit of detection was noted to be 0.42pM in the buffer system. The developed method was tested in simulated body fluid, and it was found to have a good recovery rate.

An ultrasensitive colorimetric sensor for efficient detection of Hg2+ at physiological pH

The estimation of Hg2+ in a buffer system (Tris buffer; pH 7.4) has been performed using a novel, enzyme-based, indirect detection method with the aid of non-functionalized AgNPs. The designed system has a good linear correlation (R2 = 0.9759) in the range of 1 to 10 × 10−12 M. The probe was shown to possess an extremely low limit of detection of 1.304 × 10−12 M, which is the lowest reported for the detection of Hg2+ at physiological pH (7.4). The probe was found to have good recovery rates when tested with Simulated Body Fluids (SBFs).