Sruthi Ann Alex

Combined toxicity of two crystalline phases (anatase and rutile) of Titania nanoparticles towards freshwater microalgae: Chlorella sp.

In view of the increasing usage of anatase and rutile crystalline phases of titania NPs in the consumer products, their entry into the aquatic environment may pose a serious risk to the ecosystem. In the present study, the possible toxic impact of anatase and rutile nanoparticles (individually and in binary mixture) was investigated using freshwater microalgae, Chlorella sp. at low exposure concentrations (0.25, 0.5 and 1mg/L) in freshwater medium under UV irradiation. Reduction of cell viability as well as a reduction in chlorophyll content were observed due to the presence of NPs. An antagonistic effect was noted at certain concentrations of binary mixture such as (0.25, 0.25), (0.25, 0.5), and (0.5, 0.5) mg/L, and an additive effect for the other combinations, (0.25, 1), (0.5, 0.25), (0.5, 1), (1, 0.25), (1, 0.5), and (1, 1) mg/L. The hydrodynamic size analyses in the test medium revealed that rutile NPs were more stable in lake water than the anatase and binary mixtures [at 6h, the sizes of anatase (1mg/L), rutile NPs (1mg/L), and binary mixture (1, 1mg/L) were 948.83±35.01nm, 555.74±19.93nm, and 1620.24±237.87nm, respectively]. The generation of oxidative stress was found to be strongly dependent on the crystallinity of the nanoparticles. The transmission electron microscopic images revealed damages in the nucleus and cell membrane of algal cells due to the interaction of anatase NPs, whereas rutile NPs were found to cause chloroplast and internal organelle damages. Mis-shaped chloroplasts, lack of nucleus, and starch-pyrenoid complex were noted in binary-treated cells. The findings from the current study may facilitate the environmental risk assessment of titania NPs in an aquatic ecosystem.

Simple fluorescence-based detection of Cr(III) and Cr(VI) using unmodified gold nanoparticles

We present herein a fluorescence-based method for the determination of both the trivalent and hexavalent forms of chromium in aqueous solutions using unmodified gold nanoparticles. The concept of the sensor was designed based on the aggregation of gold nanoparticles (Au NPs) by Cr(III), which results in a color change from red to blue and the appearance of a new secondary peak at 714 nm. The complexation of Au NPs by Cr(III) consequently leads to the quenching of the fluorescence intensity of Au NPs, which is proportional to the concentration of Cr(III). The Au NP aggregation upon the addition of Cr(III) is well correlated with the mean hydrodynamic size measurements and scanning electron microscopy images. The system was found to possess a good linear correlation between the chromium concentration and the degree of reduction of fluorescence intensity (R2 = 0.989) in the range of 10−7–10−3 M with an excellent limit of detection of 10−7 M (5 ppb). The prospective application of the as-designed probe for environmental sensing can be highlighted because it has been found to successfully determine the chromium concentration in real water samples. Our method has the advantage of cost-effectiveness and does not use any additional fluorophores for the sensitive detection of both the forms of chromium.

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.

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.

Etching-based transformation of dumbbell-shaped gold nanorods facilitated by hexavalent chromium and their possible application as a plasmonic sensor

The seed-mediated synthesis of anisotropic gold nanorods (AuNRs) has attracted attention due to their tunable morphology-dependent optical properties and wide range of applicability. Since the growth of nanorods can be modulated by metal ions, we have explored the Cr(VI)-assisted transformation of AuNRs. In the current investigation, the transformation of dumbbell-shaped AuNRs by Cr(VI) has been studied based on observations from UV-visible spectroscopy, transmission electron microscopy, mean hydrodynamic size measurements, and zeta potential analyses. The Cr(VI)-assisted concentration-dependent reshaping of dumbbell-shaped nanorods to shorter nanorods and spherical particles was observed with a corresponding change in their spectral properties, rod length, and zeta potential. A mechanism to understand this reshaping and etching of dumbbell-shaped nanorods into smooth rods is also proposed. The application of dumbbell-shaped AuNRs for Cr(VI) detection has been presented based on the reshaping effect observed. This method offers a detection limit of 0.071 μM with linearity in the range of 2–10 μM (R2 = 0.9978). This is the first-ever study, wherein the concentration-dependent transition of dumbbell-shaped AuNRs upon interaction with Cr(VI) was extensively investigated. This method displays good sensitivity and selectivity against most interferents and has been validated in environmental samples (lake, tap, and bore well water) with high recovery rates.

State-of-the-art strategies for the colorimetric detection of heavy metals using gold nanorods based on aspect ratio reduction

The differential polarization of the incident light along the short and long axes of AuNRs provide unique optical properties that can be used for sensing applications. Since the longitudinal surface plasmon resonance responds to variations in the local environments, it can be used as a sensitive tool for the detection of different analytes based on the modification in the aspect ratio of AuNRs caused by the preferential interaction of metal ions with particular AuNR facets. This feature can be used for the detection of metals that are toxic or important for the functioning of the body. A summarized report of the different metals detected and the current strategies devised to attain improved sensitivity and selectivity based on the variations in the AuNR aspect ratio have been discussed in this mini-review. The various etchants and conditions used and the mechanism proposed have also been elaborated. We conclude the discussion by highlighting the future prospects and the challenges that need to be overcome for the development of probes for the on-site detection of metals in the environment.

Acetylcholinesterase inhibition-based colorimetric determination of Hg2+ using unmodified silver nanoparticles

A novel, enzyme-based, indirect detection method for mercury in aqueous solutions has been developed using unmodified silver nanoparticles. The enzyme acetylcholinesterase breaks down acetylthiocholine to thiocholine, which results in the crosslinking of the silver nanoparticles. The presence of mercury in solution prevents the aggregation of the silver nanoparticles by reversibly inhibiting the active site of the enzyme. The UV-visible absorption peak at 393 nm for the unmodified silver nanoparticles was found to decrease in the presence of acetylcholinesterase and acetylthiocholine. Upon adding mercury to the system, a concentration-dependent increase in the primary peak intensity at 393 nm was noted. The designed system was found to have good linearity (R2 = 0.9799) in the range of 10 to 50 nM of mercury with an excellent limit of detection of 1.18 × 10−9 M in aqueous solution. This probe was successfully employed for the detection of mercury in spiked lakewater, groundwater, and seawater samples.

Acetylcholinesterase (AChE)-mediated immobilization of silver nanoparticles for the detection of organophosphorus pesticides

In this work, we report an enzyme-mediated biosensor with the immobilization of silver nanoparticles (AgNPs) for the detection of organophosphorus (OP) pesticides. Acetylcholinesterase (AChE) catalyzes the hydrolysis of acetylthiocholine into thiocholine, which has a thiol molecule that inhibits the growth of AgNPs. The developed method is based on the inhibition of the enzyme by malathion or trichlorfon, which blocks the generation of thiocholine and leads to the growth of AgNPs during immobilization. The efficiency of enzyme activity was determined during the addition of OP pesticides, which can be correlated to the increase in intensity of the AgNP peak at 420 nm. The increase in intensity of AgNPs was dependent on the concentrations of OPs, and thus, this method allowed low-level detection of 0.455 nM and 5.46 nM for malathion and trichlorfon, respectively, by UV-visible spectroscopy. Further, the proposed detection method was successfully utilized for the detection of OP pesticides in different spiked real samples.

Significance of surface functionalization of Gold Nanorods for reduced effect on IgG stability and minimization of cytotoxicity

Gold nanorods (AuNRs) used for biomedical applications could be encountered by biomolecules in the bloodstream, of which IgG is the most abundant antibody. With a view to mitigate their side effect on encountered proteins, the effect of Au concentration (5-40μM) and functionalization (CTAB-positive;PSS-negative; PEG-neutral) of AuNRs was investigated on the stability of a model protein, IgG (1μM). Electron microscopic images and particle size analyses indicated least aggregation behavior for PEG-AuNRs, which can be correlated to their neutral charge (from zeta potential analyses) or stearic hindrance of PEG chains. Variations in tryptophan domain were probed by UV-visible absorption and fluorescence quenching studies. Synchronous fluorescence study helped to provide information regarding variations in the hydrophobic region of IgG. The denaturation studies also indicated the stability of AuNR-IgG complex formation. These studies showed that positively charged IgG (pI: 7.8±1.0) was mostly affected by negatively charged PSS-AuNRs and least affected by PEG-AuNRs. This was verified by secondary structural investigations performed using CD and FTIR spectroscopy. For cytotoxicity studies on human lymphocytes, CTAB-AuNRs are known to show higher toxicity compared to PSS-AuNRs and PEG-AuNRs (least). Though PSS-functionalized AuNRs were shown to affect cells to a lesser degree based on the negative charge of cell membrane, they could hamper with positively charged biomolecules in the bloodstream before they reach the target, which must also be considered for choosing the right AuNR functionalization. Thus, this work indicates the effect of different AuNR functionalization on protein and cellular toxicity and stresses the necessity to use neutral particles to mitigate their side effect for theranostic applications.

A comprehensive investigation of the differential interaction of human serum albumin with gold nanoparticles based on the variation in morphology and surface functionalization

The increasing utilization of nanoparticles for biological applications necessitates the understanding of the interaction of these nanoparticles with biomolecules. Hence, the current study systematically investigated the effect of gold nanoparticle (AuNP) morphology (nanorods and nanospheres) and surface functionalization (CTAB and PEG) on a protein abundant in the blood serum, namely human serum albumin (HSA) using multiple spectroscopic techniques. The UV-visible and fluorescence spectroscopic results indicated that PEGylated AuNPs showed less ground-state complex formation and less disruption in the Trp domain when compared to CTAB-capped AuNPs, which correlated with the lower mean hydrodynamic size observed using dynamic light scattering analysis and higher activation energy required for complex formation. Further experiments have also been performed to suggest the formation of the AuNP–HSA complex. Using synchronous fluorescence, the change in the hydrophobic environment of the Trp domain was also observed to be higher for CTAB-AuNPs. Conformational stability of CTAB-AuNPs was also found to be lower in comparison to PEG-AuNPs from denaturation studies. These results were correlated with the higher secondary structural damage observed for CTAB-AuNPs compared to PEGylated counterparts using circular dichroism (CD) and FTIR analyses. The zeta potential measurements suggested that the positive charge of CTAB also becomes an additional factor that results in a higher aggregation level. Throughout the study, gold nanostructures that have rod-shaped morphology were found to cause more damage than spherical nanoparticles, but the level of protein denaturation could be considerably reduced by replacing the functionalization of the nanoparticles with PEG.