Interactions between gold, thiol and As(iii) for colorimetric sensing.

Development of a gold nanomaterial enabled colorimetric sensor method for the analysis of sodium chloride in seawater

Selective and sensitive colorimetric sensor of mercury (II) based on gold nanoparticles and 4-mercaptophenylboronic acid

Colorimetric analysis has been used to determine the biomolecules interaction in a solution with that can be indicated from color changes that can be observed either by spectrophotometer or naked eyes. Gold nanoparticles (GNP) which originally exist as pink color solution will aggregate and turn to purple color when mixed with salt solution. It was used as the basic solution and it mixed with aptamer probe and when NaCl salt solution was added to the mixture, the color changes was observed from pink to purple in the presence of HIV-1 Tat protein target. GNP are capable of adsorbing small oligonucleotides due to their propensity for electrostatic attractions, hydrophobic absorption, and covalent binding. In this experiment, GNP adsorbed the aptamer probe covering its surface preventing salt solution from interacting with GNP. Colorimetric detection is use to prove the interaction that happened in the mixing solution of aptamer and protein by observing the color changes with the presence of target and measuring the absorbance of wavelength of the solution. The wavelength shift has been observed due to changes in color. The value of wavelength for GNP observed was 522nm and shifted in wavelength can be observed in the presence of tat protein at 531.83nm while for tat peptide at 529.84nm

We report on a novel colorimetric sensor system for highly sensitive detection of formaldehyde (FA) in the gas phase. The sensor is constructed with paper towel as a substrate coated with the sulfuric acid salt of hydroxylamine ((NH2OH)2·H2SO4) together with two pH indicators, bromophenol blue and thymol blue. Upon exposure to FA, the hydroxylamine will react with the absorbed FA to form a Schiff base (H2C=N-OH), thus releasing a stoichiometric amount of sulfuric acid, which in turn induces a color change of the pH indicator. Such a color change was significantly enriched by incorporating two pH indicators in the system. With the optimized molar ratio of the two pH indicators, the color change (from brown to yellow, and to red) could become so dramatic as to be visible to the eye depending on the concentration of FA. In particular, under 80 ppb of FA (the air quality threshold set by WHO) the color of the sensor substrate changes from brown to yellow, which can even be envisioned clearly by the naked eyes. By using a color reader, the observed color change can be measured quantitatively as a function of the vapor concentration of FA, which produces a linear relationship as fitted with the data points. This helps estimate the limit of detection (LOD), to be 10 ppb under an exposure time of 10 min, which is much lower than the air quality threshold set by WHO. The reported sensor also demonstrates high selectivity towards FA with no color change observed when exposed to other common chemicals, including solvents and volatile organic compounds. With its high sensitivity and selectivity, the proposed paper-based colorimetric sensor thus developed can potentially be employed as a low-cost and disposable detection kit that may find broad application in detecting FA in indoor air and many other environments.

Colorimetric detection of Ag+ based on C[sbnd]Ag+[sbnd]C binding as a bridge between gold nanoparticles

Nowadays colorimetric detection of ascorbic acid based on enzyme-mimetic materials are mostly rely on chemically synthesized materials. Although nano enzymes prepared from natural source materials have been reported, their preparation required harsh conditions or multi steps. In this work, nanocomposite materials (MCTs) based colorimetric sensor assisted with smartphone was designed for rapid and on-site ascorbic acid (AA) detection. The MCTs can catalyze the oxidation of 3,3’,5,5’-tetramethylbenzidine (TMB) resulting in a color change from colorless to a deep blue in the presence of H2O2. The optimal catalytic conditions of pH, temperature and the material dosage are 3.0, 25°C and 3.0 mg, respectively. Due to the strong reducing property of ascorbic acid, the oxidation of TMB is inhibited so that the blue color faded. Based on the above principle, colorimetric sensor integrated with smartphone RGB mode was fabricated and exhibited a good linear detection range (1–250 µM) and low detection limit of 0.218 µM for AA detection under optimal conditions. The established method can also detect ascorbic acid (AA) in six commercial beverages with good accuracy. Therefore, this study has delivered a new cost-effective approach for the precise detection of ascorbic acid (AA) with a vast potential for various applications.

The present work aimed at synthesizing gold nanoparticles in a biological method employing fruit peel waste dumped in the environment. The peels of Garcinia mangostana (Mangostan), were collected from the nearby tourist spot during the season. The collected fruit peels were washed, dried, powder and extracted by using boiling water and acetone. The precipitated extract was dried and powdered for further use. The dried and powdered peel extract was added to the gold solution and boiled to 80 °C and the color change is observed. The color change indicates the completion of the synthesis of gold nanoparticles. The effect of pH, gold ion concentration, peel extract powder concentration, and the temperature was tested by varying the parameters. The biosynthesized nanoparticles were characterized using the UV–Vis spectrophotometer to identify the surface plasmon resonance peaks corresponding to gold nanoparticles. The bio-moieties responsible for the synthesis of gold nanoparticles were identified using the Fourier Transform Infra-Red Spectroscopy. The crystalline nature was detected by using an X-Ray Diffractometer. Atomic Force Microscope viewed the 3D surface image of the gold nanoparticle. The shape and morphology of the nanoparticle were identified by using a Field Emission Scanning Electron Microscope. The active compounds for gold nanoparticle synthesis were identified using Gas Chromatography-Mass Spectrometry. The gold nanoparticle was synthesized in various colors and used for dyeing cotton fabrics. The dyed cotton materials were exposed to various stress conditions to determine the color fastening.

Colorimetric paper sensors are used in various fields due to their convenience and intuitive manner. However, these sensors present low accuracy in practical use because it is difficult to distinguish color changes for a minute amount of analyte with the naked eye. Herein, we demonstrate that a machine learning (ML)-based paper sensor platform accurately determines the color changes. We fabricated a colorimetric paper sensor by adsorbing polyaniline nanoparticles (PAni-NPs), whose color changes from blue to green when the ambient pH decreases. Adding glucose oxidase (GOx) to the paper sensor enables colorimetric glucose detection. Target analytes (10 μL) were aliquoted onto the paper sensors, and their images were taken with a smartphone under the same conditions in a darkroom. The red-green-blue (RGB) data from the images were extracted and used to train and test three regression models: support vector regression (SVR), decision tree regression (DTR), and random forest regression (RFR). Of the three regression models, RFR performed the best at estimating pH levels (R2 = 0.957) ranging from pH 2 to 10 and glucose concentrations (R2 = 0.922) ranging from 0 to 10 mg mL-1.

Visual sensing of proteins using gold nanoparticles coated with polyphenolic glycoside

In this work, a paper-based analytical device (PAD) based on colorimetric assay using pyrrolidinyl peptide nucleic acid (acpcPNA) probe was developed as a sensor for the detection of Human papillomavirus (HPV) DNA. Dextrin-stabilized gold nanoparticles (d-AuNPs) was employed as a colorimetric reagent. The aggregation of d-AuNPs can be induced by positively charged acpcPNA generating a distinctive color change. After the hybridization of acpcPNA and DNA target, the residual acpcPNA probe can cause different degrees of the d-AuNPs aggregation, resulting in the detectable color change. The different color change before and after the introduction of the DNA target as a function of DNA concentration was quantified by analyzing the color intensity through a smartphone application. Under the optimal conditions, the linearity in the range from 1 to 1000 nM with a correlation coefficient of 0.9996 and an experimental limit of detection of 1 nM was obtained. In addition, the acpcPNA probe exhibited high selectivity for the target DNA over single-base-mismatch, two-base-mismatch, and non-complementary DNA. The proposed smartphone-based colorimetric DNA sensor offers a simple, sensitive, and selective platform for detecting HPV DNA. Therefore, this DNA sensing device can be utilized as an alternative tool for point-of-care and economical screening of HPV that is a major cause of cervical cancer.

On-site colorimetric detection of adulterated gasoline using highly reflective 1D photonic crystal sensors based on photo-crosslinked polymer–titania hybrids

A color change observable by the naked eye to indicate the content of an analyte is considered to be the most conceivable way of various sensing protocols. By taking advantage of the Förster resonance energy transfer (FRET) principles, we herein designed a dual-emission ratiometric fluorescent aptasensor for ochratoxin A (OTA) detection via a dual mode of fluorescent sensing and onsite visual screening. Amino group-modified OTA's aptamer was firstly labeled with the green-emitting CdTe quantum dots (gQDs) donor. The red-emitting CdTe QDs (rQDs) which were wrapped in the silica sphere could serve as the reference signal, while the gold nanoparticle (AuNP) acceptors were attached on the silica surface to bind with the thiolated complementary DNA (cDNA). The hybridization reaction between the aptamer and the cDNA brought gQD-AuNP pair close enough, thereby making the FRET occur in the aptasensor fabrication, while the subsequent fluorescence recovery induced by OTA was obtained in the detection procedure. Based on the red background of the wrapped rQDs, the aptasensor in response to increasing OTA displayed a distinguishable color change from red to yellow-green, which could be conveniently readout in solution even by the naked eye. Since the bioconjugations used as the aptasensor can be produced at large scale, this method can be used for in situ, rapid, or high-throughput OTA detection after only an incubation step in a homogeneous mode. We believe that this novel aptasensing strategy provides not only a promising method for OTA detection but also a universal model for detecting diverse targets by changing the corresponding aptamer.

A colorimetric array is described for the sensitive discrimination of proteins and microorganisms. Carbon dots (CDs) were prepared from citric acid and one of the amino acids glycine, lysine, serine or aspartic acid. They act as stabilizers for gold nanoparticles (AuNPs). The interactions between target protein and the CD/AuNPs induce a different aggregation behavior. This provides the basis for colorimetric discrimination of protein species and results in color changes from red/purple to purple/blue. Specific response patterns are analyzed by linear discriminant analysis. Twelve kinds of proteins with different pI and molecular weight were visually discriminated at nanomolar concentration levels. Alternatively, discrimination can be performed by measurement of the ration of absorbance at 525nm and 620nm. The discrimination sensitivity is as low as 2nM. The method can differentiate between BSA and HSA. Twelve proteins were successfully distinguished in (spiked) urine samples. The discrimination accuracy is 100% at the 500nM protein concentration level. In addition, different strains of microorganisms (E. coli O157:H7, E.coli ER2738, P. aeruginosa CICC10204; P. aeruginosa CICC21954; B.subtilis CICC10071; B.subtilis CICC10275) can be discriminated successfully via this array. Graphical abstract A CD/AuNPs-based colorimetric array sensor is proposed for the discrimination of protein, offering a discrimination sensitivity low down to 2nM. The accurate differentiations of microorganisms originated from same species are achieved.

The increasing resistance of pathogenic bacteria to the pharmaceutical compound could be a very important issue for public health. A sensitive, fast and accurate approaches for bacteria detection are going to be needed to enhance the effectiveness of treatment. This study aimed to develop a colorimetric sensor based on allylamine-conjugated polyacrylic acid and gold nanoparticles (AuNPs- AAm-PAA) for bacteria detection. Allylamine was conjugated to the polyacrylic acid and mixed with the gold colloid to form a self-assembled AuNPs-AAm-PAA. The polymer-modified gold nanoparticles were characterized for the particle size, zeta potential, reversibility using TEM, particle size analyzer, and UV-Vis spectrophotometer, respectively. The system was observed to detect bacteria and observed by the naked eye, UV-Vis spectrophotometer, and TEM. The results showed that AuNPs-PAA-AAm solution showed an absorbance peak at 530 nm, AuNPs particle size of 25.67 nm ± 2.68, and zeta potential -31.4 ± 4.5 mV. Introduction of bacteria into the Au-AAm-PAA solution resulted in a color change from red to purple at concentrations of 6x108 CFU/mL of Staphylococcus aureus and Escherichia coli. TEM analysis revealed the binding of aggregated gold nanoparticles around the outer membrane of bacteria. The sensor system based on conjugated polymer and gold nanoparticles demonstrated a promising way for bacteria detection.

Surface ligands play a critical role in controlling and defining the properties of colloidal nanocrystals. These aspects have been exploited to design nanoparticle aggregation-based colorimetric sensors. Here, we coated 13-nm gold nanoparticles (AuNPs) with a large library of ligands (e.g., from labile monodentate monomers to multicoordinating macromolecules) and evaluated their aggregation propensity in the presence of three peptides containing charged, thiolate, or aromatic amino acids. Our results show that AuNPs coated with the polyphenols and sulfonated phosphine ligands were good choices for electrostatic-based aggregation. AuNPs capped with citrate and labile-binding polymers worked well for dithiol-bridging and π-π stacking-induced aggregation. In the example of electrostatic-based assays, we stress that good sensing performance requires aggregating peptides of low charge valence paired with charged NPs with weak stability and vice versa. We then present a modular peptide containing versatile aggregating residues to agglomerate a variety of ligated AuNPs for colorimetric detection of the coronavirus main protease. Enzymatic cleavage liberates the peptide segment, which in turn triggers NP agglomeration and thus rapid color changes in <10 min. The protease detection limit is 2.5 nM.