Selective recognition of fumarate from maleate with a gold nanoparticle-based colorimetric sensing system

A colorimetric sensing system of gold nanoparticles functionalized with carboxylate-binding units of o-(trifluoroacetyl)carboxanilide is described, which selectively recognizes a trans-dicarboxylate (fumarate) from its cis-isomer (maleate) and several dicarboxylates through inter-particle cross-linking, resulting in an apparent color change from red to purple.     

Label-free aptamer-based colorimetric detection of mercury ions in aqueous media using unmodified gold nanoparticles as colorimetric probe

We report a simple and sensitive aptamer-based colorimetric detection of mercury ions (Hg²⁺) using unmodified gold nanoparticles as colorimetric probe. It is based on the fact that bare gold nanoparticles interact differently with short single-strand DNA and double-stranded DNA. The anti-Hg²⁺ aptamer is rich in thymine (T) and readily forms T–Hg²⁺–T configuration in the presence of Hg²⁺. By measuring color change or adsorption ratio, the bare gold nanoparticles can effectively differentiate the Hg²⁺-induced conformational change of the aptamer in the presence of a given salt with high concentration. The assay shows a linear response toward Hg²⁺ concentration through a five-decade range of 1 × 10⁻⁴ mol L⁻¹ to 1 × 10⁻⁹ mol L⁻¹. Even with the naked eye, we could identify micromolar Hg²⁺ concentrations within minutes. By using the spectrometric method, the detection limit was improved to the nanomolar range (0.6 nM). The assay shows excellent selectivity for Hg²⁺ over other metal cations including K⁺, Ba²⁺, Ni²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Mg²⁺, Ca²⁺, Zn²⁺, Al³⁺, and Fe³⁺. The major advantages of this Hg²⁺ assay are its water-solubility, simplicity, low cost, visual colorimetry, and high sensitivity. This method provides a potentially useful tool for the Hg²⁺ detection.     

Enzymatic cleavage and mass amplification strategy for small molecule detection using aptamer-based fluorescence polarization biosensor

Fluorescence polarization (FP) assays incorporated with fluorophore-labeled aptamers have attracted great interest in recent years. However, detecting small molecules through the use of FP assays still remains a challenge because small-molecule binding only results in negligible changes in the molecular weight of the fluorophore-labeled aptamer. To address this issue, we herein report a fluorescence polarization (FP) aptamer assay that incorporates a novel signal amplification strategy for highly sensitive detection of small molecules. In the absence of adenosine, our model target, free FAM-labeled aptamer can be digested by nuclease, resulting in the release of FAM-labeled nucleotide segments from the dT-biotin/streptavidin complex with weak background signal. However, in the presence of target, the FAM-labeled aptamer–target complex protects the FAM-labeled aptamer from nuclease cleavage, allowing streptavidin to act as a molar mass amplifier. The resulting increase in molecular mass and FP intensity of the aptamer–target complex provides improved sensitivity for concentration measurement. The probe could detect adenosine from 0.5 μM to 1000 μM, with a detection limit of 500 nM, showing that the sensitivity of the probe is superior to aptamer-based FP approaches previously reported for adenosine. Importantly, FP could resist environmental interferences, making it useful for complex biological samples without any tedious sample pretreatments. Our results demonstrate that this dual-amplified, aptamer-based strategy can be used to design fluorescence polarization probes for rapid, sensitive, and selective measurement of small molecules in complicated biological environment.     

Homogeneous, unmodified gold nanoparticle-based colorimetric assay of hydrogen peroxide

An unmodified gold nanoparticle-based colorimetric assay system in homogeneous format has been developed using hydrogen peroxide (H₂O₂) as a model analyte. H₂O₂ is added to o-phenylenediamine/horseradish peroxidase solution, and allowed to react for 10 min. Then, unmodified gold nanoparticles that serve as “reaction indicators” are added to the reaction solution. The resulting mixture color changes dramatically from red to blue. The reason is that azoaniline, a horseradish peroxidase-catalyzed oxidation product, induces the nanoparticle aggregation. Using this approach, H₂O₂ can be semiquantitatively determined over the concentration range of ∼4 orders of magnitude by the naked eye. If the observed peak intensity at 420 nm is used for the construction of the calibration plot, hydrogen peroxide can be accurately determined down to concentration levels of 1.3 × 10⁻⁶ M. Compared with the conventional electrochemical protocol, this sensing system offers several important advantages: (1) ability to be monitored by the naked eye, (2) avoiding the need of surface modification of electrodes or gold nanoparticles and (3) detection in homogeneous solution. It is worthy of note that this efficient and convenient strategy is also suitable for the detection of other species, such as glucose and cholesterol.     

Specific ionic effect for simple and rapid colorimetric sensing assays of amino acids using gold nanoparticles modified with task-specific ionic liquid

In this study, a novel task-specific ionic liquid functionalized gold nanoparticle (TSIL-GNP) was successfully prepared and applied in the recognition of amino acids. Particularly, the surface of GNP was modified with the ionic liquid containing carbamido and ester group via thiol, which was characterized by Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The stability of this material in aqueous solution improves apparently and can remain unchanged for more than three months. The effect of pH was also discussed in this study. Attractive ionic interaction would effectively weaken intensity of the covalent coupling between the metal ion and the functional groups of amino acids. Thus, TSIL-GNP was successfully applied to recognizing serine, aspartic acid, lysine, arginine, and histidine in the presence of Cu²⁺ through distinctive color changes. Suspension would be generated once a spot of cysteine was added into the GNPs solution. Results indicated that it had a good linear relationship between extinction coefficients and concentration of amino acids in a wide range of 10⁻³–10⁻⁶ M. Moreover, the proposed strategy was successfully used to analyze the histidine in urinary samples. In brief, TSIL-GNP is a suitable substrate for discrimination of five amino acids in a rapid and simple way without sophisticated instruments.     

Visual detection of cancer cells by colorimetric aptasensor based on aggregation of gold nanoparticles induced by DNA hybridization

A simple but highly sensitive colorimetric method was developed to detect cancer cells based on aptamer–cell interaction. Cancer cells were able to capture nucleolin aptamers (AS 1411) through affinity interaction between AS 1411 and nucleolin receptors that are over expressed in cancer cells, The specific binding of AS 1411 to the target cells triggered the removal of aptamers from the solution. Therefore no aptamer remained in the solution to hybridize with complementary ssDNA-AuNP probes as a result the solution color is red. In the absence of target cells or the presence of normal cells, ssDNA-AuNP probes and aptamers were coexisted in solution and the aptamers assembled DNA-AuNPs, produced a purple solution. UV–vis spectrometry demonstrated that this hybridization-based method exhibited selective colorimetric responses to the presence or absence of target cells, which is detectable with naked eye. The linear response for MCF-7 cells in a concentration range from 10 to 10⁵ cells was obtained with a detection limit of 10 cells. The proposed method could be extended to detect other cells and showed potential applications in cancer cell detection and early cancer diagnosis.     

Rapid and sensitive detection of cholera toxin using gold nanoparticle-based simple colorimetric and dynamic light scattering assay

Herein, a rapid and simple gold nanoparticle based colorimetric and dynamic light scattering (DLS) assay for the sensitive detection of cholera toxin has been developed. The developed assay is based on the distance dependent properties of gold nanoparticles which cause aggregation of antibody-conjugated gold nanoparticles in the presence of cholera toxin resulting discernible color change. This aggregation induced color change caused a red shift in the plasmon band of nanoparticles which was measured by UV–Vis spectroscopy. In addition, we employed DLS assay to monitor the extent of aggregation in the presence of different concentration of cholera toxin. Our assay can visually detect as low as 10 nM of cholera toxin which is lower than the previously reported colorimetric methods. The reported assay is very fast and showed an excellent specificity against other diarrhetic toxins. Moreover, we have demonstrated the feasibility of our method for cholera toxin detection in local lake water.     

A gold nanoparticle-based colorimetric sensing ensemble for the colorimetric detection of cyanide ions in aqueous solution

A colorimetric sensing ensemble was prepared by mixing readily prepared adenosine triphosphate (ATP)-stabilized AuNPs with Cu²⁺-phenanthroline complexes. The sensing mechanism of the ensemble was examined by UV-vis spectrometry and transmission electron microscopy. The studies showed that the Cu²⁺-phenanthroline complex was converted to free phenanthroline when exposed to cyanide anions and the free phenanthroline caused the ATP-stabilized AuNPs to aggregate, which in turn, resulted in a visible color change in the AuNP solution. The ensemble could detect cyanide ions in aqueous solution at physiological pH, either spectrophotometrically or visually, with high selectivity toward cyanide anions over a range of mono- and di-anions commonly found in biological and environmental systems. This sensing ensemble also allows a quantitative assay of the analyte in a neutral aqueous solution, down to a concentration of 10⁻⁵ M.     

A selective,sensitive and label-free visual assay of fructose using anti-aggregation of gold nanoparticles as a colorimetric probe

A new convenient colorimetric sensor for fructose based on anti-aggregation of citrate-capped gold nanoparticles(Au NPs) is presented. 4-Mercaptophenylboronic acid(MPBA) induces the aggregation of Au NPs, leading to a color change from red to blue. Fructose as a potent competitor has strong affinity for MPBA and a borate ester is formed between MPBA and fructose. There is an obvious color change from blue to red with increasing the concentration of fructose. The anti-aggregation effect of fructose on Au NPs was seen by the naked eye and monitored by UV–vis spectra. Our results showed that the absorbance ratio(A_(519)/A_(640)) was linear with fructose concentration in the range of 0.032–0.96 μmol/L(R~2= 0.996), with a low detection limit of 0.01 μmol/L(S/N = 3). Notably, a highly selective recognition of fructose was shown against other monosaccharide and disaccharide(glucose, mannose, galactose,lactose and saccharose). With anti-aggregation assays higher selectivity is achievable. The results of this work provide a rapid method for evaluating the quantitative analysis of fructose in human plasma at physiologically meaningful concentrations and at neutral pH. The proposed procedure can be used as an efficient method for the precise and accurate determination of fructose.     

Triarylcarbinol functionalized gold nanoparticles for the colorimetric detection of nerve agent simulants

Gold nanoparticles functionalized with a triarylcarbinol derivative have been used as colorimetric molecular probes for the naked-eye detection of the nerve agent simulants DCNP and DFP. The detection process is based on the compensation of charges at the surface of the nanoparticles which triggers their aggregation in solution with the resulting change in their plasmon band.     

Plasmonic-based colorimetric and spectroscopic discrimination of acetic and butyric acids produced by different types of Escherichia coli through the different assembly structures formation of gold nanoparticles

We present a plasmonic-based strategy for the colourimetric and spectroscopic differentiation of various organic acids produced by bacteria. The strategy is based on our discovery that particular concentrations of dl-lactic, acetic, and butyric acids induce different assembly structures, colours, and optical spectra of gold nanoparticles. We selected wild-type (K-12 W3110) and genetically-engineered (JHL61) Escherichia coli (E. coli) that are known to primarily produce acetic and butyric acid, respectively. Different assembly structures and optical properties of gold nanoparticles were observed when different organic acids, obtained after the removal of acid-producing bacteria, were mixed with gold nanoparticles. Moreover, at moderate cell concentrations of K-12 W3110 E. coli, which produce sufficient amounts of acetic acid to induce the assembly of gold nanoparticles, a direct estimate of the number of bacteria was possible based on time-course colour change observations of gold nanoparticle aqueous suspensions. The plasmonic-based colourimetric and spectroscopic methods described here may enable onsite testing for the identification of organic acids produced by bacteria and the estimation of bacterial numbers, which have applications in health and environmental sciences.     

Label-free colorimetric sensing of ascorbic acid based on Fenton reaction with unmodified gold nanoparticle probes and multiple molecular logic gates

A label-free strategy based on Fenton reaction with unmodified gold nanoparticles (AuNPs) as probe is demonstrated for ascorbic acid (AA) sensing. AuNPs is stable in the presence of single stranded DNA (ssDNA) which prevents salt-induced aggregation of AuNPs in solution. The hydroxyl free radicals generated by Fenton reaction lead to ssDNA cleavage into different sequence fragments which induce aggregation of AuNPs to produce a red-to-blue color change. As an efficient biological antioxidant, AA could effectively scavenge free radicals to avoid the cleavage of ssDNA, so that it prevents color change of the AuNPs solution. Thus, the color change of AuNPs in the presence and absence of AA provides a new approach for the detection of AA. The absorbance ratio at two wavelengths, A₆₇₀/A₅₂₀, decreases linearly with AA content within 1–15 μM, giving rise to a detection limit of 0.3 μM and a RSD of 2.8% (10 μM). The color display of AuNPs solution makes it feasible for the estimation of AA content by naked eye visualization. Moreover, based on Fenton reaction and unmodified gold nanoparticles, a multiple logic gate system includes two logic operations, i.e., INHIBIT and NOR, has been designed with small molecules (AA, l-cysteine, glutathione) as inputs and the colorimetric changes of AuNPs solution as outputs.     

A non-aggregation colorimetric assay for thrombin based on catalytic properties of silver nanoparticles

In this paper, we developed a simple and rapid colorimetric assay for protein detection based on the reduction of dye molecules catalyzed by silver nanoparticles (AgNPs). Aptamer-modified magnetic particles and aptamer-functionalized AgNPs were employed as capture and detection probes, respectively. Introduction of thrombin as target protein could form a sandwich-type complex involving catalytically active AgNPs, whose catalytic activity was monitored on the catalytic reduction of rhodamine B (RhB) by sodium borohydride (NaBH₄). The amount of immobilized AgNPs on the complex increased along with the increase of the thrombin concentration, thus the detection of thrombin was achieved via recording the decrease in absorbance corresponding to RhB. This method has adopted several advantages from the key factors involved, i.e., the sandwich binding of affinity aptamers contributed to the increased specificity; magnetic particles could result in rapid capture and separation processes; the conjugation of AgNPs would lead to a clear visual detection. It allows for the detection limit of thrombin down to picomolar level by the naked eye, with remarkable selectivity over other proteins. Moreover, it is possible to apply this method to the other targets with two binding sites as well.     

Angle-independent responsive organogel retroreflective structural color film for colorimetric sensing of humidity and organic vapors

The angle dependence of photonic crystals (PCs) dramatically limits their practical applications in the colorimetrical sensing of humidity and volatile organic compound (VOC) vapors. In addition, it is challenging for inverse opal PCs to colorimetrically distinguish between vapors with similar refractive indices. Different from the mechanism of PC-based sensors, here, we report an angle-independent polyacrylamide (PAAm) organogel structural color film based on the mechanisms of retroreflection, total internal reflection (TIR) and interference with a shape similar to a single-sided “egg waffle”. During the process of responding to humidity and VOC vapors, the color of the film remains angle-independent in the normal angle range of 0° to 45° under coaxial illumination and observation conditions. At the same time, the film can colorimetrically distinguish between vapors with similar refractive indices, such as methanol and ethanol, which is mainly due to the differences in their polarity and solubility parameters. The film shows good stability, reversibility and selectivity when exposed to vapors. A colorimetric sensor with a new response mechanism is proposed and has the potential to effectively distinguish between vapors with similar refractive indices. Furthermore, this responsive retroreflective structural color film (RRSCF) provides a universal strategy to develop targeted angle-independent structural color sensors by selecting optimized materials.     

Novel,highly selective detection of Cr(III) in aqueous solution based on a gold nanoparticles colorimetric assay and its application for determining Cr(VI)

A simple, rapid, sensitive and field-portable colorimetric technique for the determination of Cr(III) in aqueous solution based on an aggregation-induced color transition of gold nanoparticles (AuNPs) has been developed. AuNPs were first functionalized with a dithiocarbamate-modified N-benzyl-4-(pyridin-4-ylmethyl)aniline ligand (BP-DTC). Chelation of Cr(III) by several of these ligands, bound to different nanoparticles, led to nanoparticle aggregation in solution. This gave rise to a color change from wine-red to blue that was discernible by the naked eye and an easily measurable alteration in the extinction spectrum of the particles. The method could be used to determine Cr(III) with a detection limit of 31 ppb. Furthermore, selective detection of trace Cr(III) in aqueous solution in the presence of 12 other transition metal ions has been achieved. Toward the goal of practical applications, the sensor has been further evaluated with a view to monitoring Cr(III) in nutritional supplements and the blood of diabetes patients and also applied in the indirect determination of Cr(VI) in waste water.     

Ag@Au core/shell triangular nanoplates with dual enzyme-like properties for the colorimetric sensing of glucose

Due to the serious harm of diabetes to human health, development of sensitive assays for glucose level is of high significance for early prevention and treatment of diabetes. Currently, most conventional enzyme-based glucose sensors suffer from high cost and low stability due to the inherent defects of natural enzymes. Herein, we develop a pure nanozyme-based glucose detection method using Ag@Au core/shell triangular nanoplates (TNPs), which combines glucose oxidase (GOD)- and horseradish peroxidase (HRP)-like activities of the Au shell and inherent plasmonic properties of Ag TNPs. The sensing mechanism is based on the fact that the Au shell possessed GOD-like activity, enabling the oxidation of glucose to produce H₂O₂, which can further etch the silver core, leading to the decrease of absorbance at 800 nm and the color change from blue to colorless. Compared with the previous nanozymes-based glucose sensors, our method avoids the use of enzymes and organic chromogenic agent. Moreover, the stability of the Ag@Au core/shell TNPs is much better than that of Ag TNPs due to the protection by the coating of the Au shell. This method was successfully applied to the detection of urine samples from patients with diabetes, indicating its practical applicability for real sample analysis.     

A label-free colorimetric detection of lead ions by controlling the ligand shells of gold nanoparticles

We have developed a simple, colorimetric and label-free gold nanoparticle (Au NP)-based probe for the detection of Pb²⁺ ions in aqueous solution, operating on the principle that Pb²⁺ ions change the ligand shell of thiosulfate (S₂O₃²⁻)-passivated Au NPs. Au NPs reacted with S₂O₃²⁻ ions in solution to form Au⁺·S₂O₃²⁻ ligand shells on the Au NP surfaces, thereby inhibiting the access of 4-mercaptobutanol (4-MB). Surface-assisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) and inductively coupled plasma mass spectrometry (ICP-MS) measurements revealed that PbAu alloys formed on the surfaces of the Au NPs in the presence of Pb²⁺ ions; these alloys weakened the stability of the Au⁺·S₂O₃²⁻ ligand shells, enhancing the access of 4-MB to the Au NP surfaces and, therefore, inducing their aggregation. As a result, the surface plasmon resonance (SPR) absorption of the Au NPs red-shifted and broadened, allowing quantitation of the Pb²⁺ ions in the aqueous solution. This 4-MB/S₂O₃²⁻-Au NP probe is highly sensitive (linear detection range: 0.5-10 nM) and selective (by at least 100-fold over other metal ions) toward Pb²⁺ ions. This cost-effective sensing system allows the rapid and simple determination of the concentrations of Pb²⁺ ions in real samples (in this case, river water, Montana soil and urine samples).     

Nanoengineered micro gold shells for LDI-TOF analysis of small molecules

This paper reports on analyses of small molecules with laser desorption/ionization time of flight (LDI-TOF) mass spectrometry (MS) using nanostructure-embedded micro gold shells (μAuSs). The mass analyses of amino acids, sugars, peptides, and their mixtures gave apparent mass peaks for analytes without any significant background interferences. μAuSs afforded a better limit of detection (LOD) and a higher signal-to-noise ratio than gold nanoparticles, which are commonly used for LDI-TOF analysis of small molecules. We believe μAuSs have advantages in terms of simplicity, detection limit, and reproducibility, and therefore, they constitute a significant addition to the organic matrix-free analytical tools that are currently in wide use.