Quantitative colorimetric sensing of heavy metal ions via analyte-promoted growth of Au nanoparticles with timer or smartphone readout

This work describes two new colorimetric nanosensors for label-free, equipment-free quantitative detection of nanomolar copper (II) (Cu²⁺) and mercury (II) (Hg²⁺) ions. Both are based on the analyte-promoted growth of Au nanoparticles (AuNPs) from the reduction of chloroauric acid by 4-morpholineethanesulfonic acid. For the Cu²⁺ nanosensor, the analyte can accelerate such a redox system to rapidly form a red solution containing dispersed, uniform, spherical AuNPs that is related to these particles' surface plasmon resonance property. For the Hg²⁺ nanosensor, on the other hand, a blue mixture consisting of aggregated, ill-defined AuNPs with various sizes can be created, showing a significantly enhanced Tyndall effect (TE) signal (in comparison with that produced in the red solution of AuNPs). By using a timer and a smartphone to quantitatively measure the time of producing the red solution and the TE intensity (i.e., the average gray value of the corresponding image) of the blue mixture, respectively, the developed nanosensors are well demonstrated to achieve linear ranges of 6.4 nM to 100 μM and 6.1 nM to 1.56 μM for Cu²⁺ and Hg²⁺, respectively, with detection limits down to 3.5 and 0.1 nM, respectively. The acceptable recovery results obtained from the analysis of the two analytes in the complex real water samples including drinking water, tap water, and pond water ranged from 90.43 to 111.56%.

Plasmonic nanogels for naked-eye sensing of food adulterants

Cellulose based nanoplasmonic sensors gained immense attention for various applications due to their advantageous physicochemical characteristics such as ease of fabrication, low density, chirality, surface functionalization and disposal. Herein, a hydrogel based nanoplasmonic sensor probe was fabricated and evaluated for the detection of melamine (MA). Plasmonic nanomaterials (AuNPs) were synthesized by the redox reaction using a dual reducing agent (β-cyclodextrin (βCD) and citrate). The physicochemical characteristics of the synthesized AuNPs were extensively determined by various spectroscopic and microscopic techniques. The colorimetric sensing of melamine (MA) was carried out in solution and hydrogel phases. Upon the addition of MA, AuNPs tend to aggregate and exhibit color changes from orange-red to purple due to surface plasmon resonance (SPR) coupling. This nanosensor probe showed high selectivity and sensitivity for detection of MA with a detection limit of 3 × 10^-7 M. Plasmonic hydrogels were prepared using the cellulose acetate (CA) polymer and optimized for stability and interaction with melamine. The βCD-citrate stabilized AuNPs showed color changes with the CA hydrogels. The hydrogel-based sensor probe exhibits similar characteristics with respect to the selective and sensitive detection of MA under optimized conditions. The fabricated nanoreactor based sensor probe has high potential for food sensor applications.

The selective detection of Fe3+ ions using citrate-capped gold nanoparticles

Citrate is a ubiquitous biological molecule that functions as Fe³⁺ chelators in some bacteria and the blood plasma of humans. Inspired by the strong affinity between citrate and Fe³⁺, a colorimetric Fe³⁺ probe based on citrate-capped AuNPs without any additional modification was designed. Citrate-capped AuNPs with a diameter of 22 nm were applied to detect Fe³⁺ without other reagents' assistance. This easily-prepared and low-cost colorimetric sensor exhibited good selectivity towards Fe³⁺ among common metal ions, a good linear relationship in the range of 0.1-0.8 μM of Fe³⁺ and quick response time of 10 min.

Colorimetric determination of L-cysteine in milk samples with surface functionalized silver nanoparticles

A simple, selective and sensitive method is proposed for determination of cysteine (Cys) in milk samples using ionic liquid functionalized silver nanoparticles (ILs-AgNPs) as a colorimetric probe. ILs-AgNPs was synthesized by simple reduction method using silver nitrate as a precursor and sodium borohydride as a reducing agent and functionalized with ILs to prevent particles from self-aggregation. The sensing mechanism has been dependent on the color change of ILs-AgNPs and red shift of absorption band from 395 nm to 560 nm in the visible region, which is found proportional to the concentration of target analyte in sample. ILs-AgNPs was characterized in absence and presence of Cys by UV-vis, Fourier transform-infrared (FTIR) spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The linear range was acquired in the range of 0-100 ng mL^-1, with correlation coefficient (R²) of 0.996 and limit of detection (LOD) of 4.0 nM. The binding mechanism and interactions between Cys and ILs-AgNPs was confirmed by calculating the binding constant and thermodynamic parameters such as enthalpy (∆H), entropy (∆S) and Gibb's free energy (∆G). The use of ILs-AgNPs exhibited high colorimetric selectivity for Cys in milk samples in presence of other amino acids. This proposed strategy possessed the advantages of simplicity and selectivity, hence is applied for analysis of Cys in milk samples.

Novel double layer film composed of reduced graphene oxide and Rose Bengal dye: design, fabrication and evaluation as an efficient chemosensor for silver(i) detection

A novel silver-chemosensor fabricated with reduced graphene oxide and Rose Bengal (RB) based on the interaction of Ag⁺ and RB.

Naked eye and spectrophotometric detection of chromogenic insecticide in aquaculture using amine functionalized gold nanoparticles in the presence of major interferents

Detection of a chromogenic insecticide, malachite green (MG) using 3,5-diamino-1,2,4-triazole capped gold nanoparticles (DAT-AuNPs) by both naked eye and spectrophotometry was described in this paper. The DAT-AuNPs were prepared by wet chemical method and show absorption maximum at 518nm. The zeta potential of DAT-AuNPs was found to be -39.9mV, suggesting that one of the amine groups of DAT adsorbed on the surface of AuNPs and the other amine group stabilizes the AuNPs from aggregation. The wine red color DAT-AuNPs changes to violet while adding 25μM MG whereas the absorption band at 518nm was increased and shifted towards longer wavelength. However, addition of 70μM MG leads to the aggregation of DAT-AuNPs. This is due to strong electrostatic interaction between ammonium ion of MG and the free amine group of DAT. Based on the color change and shift in SPR band, 25 and 5μM MG can be easily detected by naked eye and spectrophotometry. The DAT-AuNPs show high selectivity towards MG even in the presence of 5000-fold higher concentrations of common interferents. The practical application was successfully demonstrated by determining MG in fish farm water.

Ultra-sensitive fluorescence determination of chromium(vi) in aqueous solution based on selectively etching of protein-stabled gold nanoclusters

In this work, we have developed a simple, cost-effective and sensitive fluorescent method for the selective determination of chromium(vi) ions (Cr(vi)) in aqueous solution.

Enzyme-Free Amplification by Nano Sticky Balls for Visual Detection of ssDNA/RNA Oligonucleotides

Visual detection of nucleic acids provides simple and rapid screening for infectious diseases or environmental pathogens. However, sensitivity is the current bottleneck, which may require enzymatic amplification for targets in low abundance and make them incompatible with detection at resource-limited sites. Here we report an enzyme-free amplification that provides a sensitive visual detection of ssDNA/RNA oligonucleotides on the basis of nano "sticky balls". When target oligonucleotides are present, magnetic microparticles (MMPs) and gold nanoparticles (AuNPs) were linked together, allowing the collection of AuNPs after magnetic attraction. Subsequently, the collected AuNPs, which carry many oligonucleotides, were used as the sticky balls to link a second pair of MMPs and polymer microparticles (PMPs). Thus, because the magnetic field can attract the MMPs as well as the linked PMPs to the sidewall, the reduction of suspended PMPs yields a change of light transmission visible by the naked eye. Our results demonstrate that the limit of detection is 10 amol for ssDNAs (228 fM in 45 μL) and 75 amol for ssRNAs (1.67 pM in 45 μL). This method is also compatible with the serum environment and detection of a microRNA, miR-155, derived from human breast cancer cells. With significantly improved sensitivity for visual detection, it provides great potential for point-of-care applications at resource-limited sites.

Fenton-like reaction-mediated etching of gold nanorods for visual detection of Co(2+)

We have proposed a Fenton-like reaction-mediated etching of gold nanorods and applied it to the sensitive visual detection of Co(2+) ions. With the presence of bicarbonate (HCO3(-)) and hydrogen peroxide(H2O2), Co(2+) ions trigger a Fenton-like reaction, resulting in the generation of superoxide radical (O2(•-)). As a result, the gold nanorods are gradually etched by O2(•-) in the presence of SCN(-), accompanied by an obvious color change from green to red. The gold nanorods etching process preferentially occurs along the longitudinal direction, which is observed by transmission electron microscope. The etching mechanism is carefully proved by investigating the effects of different radical scavengers (e.g., dimethyl sulfoxide). The auto-oxidation of hydroxylamine assay further confirms the mechanism. Then, the main factors, including reactants concentrations, temperature, and incubation time, are specifically investigated. Under optimized conditions, we get an excellent sensing performance for Co(2+) with a lower detection limit of 1.0 nM via a spectrophotometer and a visual detection limit of 40 nM. In addition, this principle may provide a new concept of "intermediate-mediated etching of nanoparticles" for sensing.

Sensitive and highly selective determination of vitamin B1 in the presence of other vitamin B complexes using functionalized gold nanoparticles as fluorophore

This work describes the spectrofluorimetric determination of thiamine in the presence of vitamin B complexes using 4-amino-6-hydroxy-2-mercaptopyrimidine as fluorophore. The detection limit was found to be 6.8 fM L⁻¹ (S/N = 3).

Visual detection of nucleic acids based on Mie scattering and the magnetophoretic effect

Visual detection of nucleic acid biomarkers by magnetically attracted microparticles that change solution turbidity via Mia scattering.

Pyridoxal derivative functionalized gold nanoparticles for colorimetric determination of zinc(ii) and aluminium(iii)

This investigation presents the synthesis of a thiol derivativeLby one step condensation of pyridoxal with 4-aminothiophenol, and its functionalization on citrate capped AuNPs.

Cu(II)-coordinated GpG-duplex DNA as peroxidase mimetics and its application for label-free detection of Cu2+ ions

Herein a facile method is proposed to construct DNA-based peroxidase mimetics simply assembled by polymorphic DNA and Cu(2+) ions. The Cu(II)-catalyzed oxidation of TMB in the presence of H2O2 can be significantly accelerated through Cu(II)-coordination with DNA scaffolds, of which a colorimetric change can be discerned by naked-eye. The reaction rates of DNA-Cu(II) complexes are directly associated with sequence composition as well as the secondary structure of DNA scaffold, e.g., the reaction rate decreases in the following order: GpG-duplex ≈ G-rich coil > G-quadruplex > C-rich coil > i-motif. It is the first report to explore a colorimetric Cu(2+) sensing system on the basis of peroxidase mimicking activities of polymorphic DNA-Cu(II) complexes. One of our most intriguing results is that the GpG-duplex DNA demonstrates the ability to sense Cu(2+) ions in aqueous solution without significant interference from other metal ions. The Cu(2+) detection limit of 1.2 nM is achieved with a linear response range of 1.2-100 nM, and the developed sensing system is potentially applicable for quantitative determination of Cu(2+) in drinking water samples.

A turn-on highly selective and ultrasensitive determination of copper (II) in an aqueous medium using folic acid capped gold nanoparticles as the probe

This paper describes a 'turn-on' fluorescent determination of Cu(II) in an aqueous medium using folic acid capped gold nanoparticles (FA-AuNPs) as the probe. The FA-AuNPs were synthesized by the wet chemical method and were characterized by UV-visible, fluorescence, HR-TEM, XRD, zeta potential, and DLS techniques. The FA-AuNPs show an absorption maximum at 510 nm and an emission maximum at 780 nm (λ(ex): 510 nm). On adding 10 μM Cu(II), the wine-red color of FA-AuNPs changed to purple and the absorbance at 510 nm decreased. The observed changes were ascribed to the aggregation of AuNPs. This was confirmed by DLS and HR-TEM studies. Interestingly, the emission intensity of FA-AuNPs was enhanced even in the presence of a picomolar concentration of Cu(II). Based on the enhancement of the emission intensity, the concentration of Cu(II) was determined. The FA-AuNPs showed an extreme selectivity towards the determination of 10 nM Cu(II) in the presence of 10,000-fold higher concentration of interferences except EDTA and the carboxylate anion. A good linearity was observed from 10 × 10(-9) to 1 × 10(-12) M Cu(II), and the detection limit was found to be 50 fM l(-1) (S/N = 3). The proposed method was successfully applied to determine Cu(II) in real samples. The results obtained were validated with ICP-AES.

Surface-plasmon-based colorimetric detection of Cu(II) ions using label-free gold nanoparticles in aqueous thiosulfate systems

We report colorimetric, label-free and non-aggregation-based gold nanoparticle (AuNP) probes for the highly selective detection of Cu(II) ions in aqueous environments. This detection scheme relies on the ability of Cu(II) ions to catalyze the leaching of gold at room temperature in the presence of thiosulfate species and ammonia. This simple and cost-effective probe provides rapid detection of Cu(II) ions at concentrations as low as 10 ppm. A similar detection method using AuNPs in ammonia-free thiosulfate solution is also viable at moderate reaction temperature (50 °C). The ammonia-free method also leads to marked damping and red-shifting of the surface plasmon resonance signal of the AuNP dispersion. The two methods clearly differ in the nature of the surface plasmon damping phenomenon, and their working mechanisms are plausibly explained based on the experimental investigations.