Colorimetric aggregation assay for silver(I) based on the use of aptamer modified gold nanoparticles and C-Ag(I)-C interaction

The authors describe a gold nanoparticle (AuNP) based aggregation assay for colorimetric determination of silver ions. The detection scheme is based on the release of aptamers from the surface of AuNPs that is triggered by the formation of C-Ag(I)-C links. In the absence of Ag(I) ions, the aptamers are readily adsorbed on the surface of the AuNPs. This prevents the aggregation of AuNPs and warrants the stability of the red colloidal solution at high salt concentration. In the presence of Ag(I) ions, the aptamers are released from the surface of AuNPs due to binding to Ag(I). Hence, salt-induced aggregation of AuNPs will occur which is accompanied by a gradual color change from red to blue. The color change occurs in the 1 to 500 nM Ag(I) concentration range, and the detection limit is 0.77 nM. The method was successfully applied to the determination of Ag(I) in spiked tap water samples.

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Graphical abstract Schematic of a gold nanoparticle-based aggregation assay for colorimetric determination of silver ions. Visual quantitation also is posssible due to a gradual color change from red to blue.

     

Colorimetric detection of apoptosis based on caspase-3 activity assay using unmodified gold nanoparticles

A new label-free method for the detection of apoptosis was proposed based on colorimetric assay of caspase-3 activity using an unlabeled Asp-Glu-Val-Asp (DEVD)-containing peptide substrate and unmodified gold nanoparticles (AuNPs).     

Colorimetric determination of the pesticide chlorothalonil based on the aggregation of gold nanoparticles

A method is described for the determination of the pesticide chlorothalonil (CLT). It is based on the finding that citrate-capped gold nanoparticles (AuNPs) undergo aggregation on exposure to chlorothalonil. This is accompanied by a visually detectable color change from wine red to blue. The effect is due to the interaction of the cyano group of chlorothalonil with gold nanoparticles. The assay may also be performed by using a spectrometer. The ratio of absorbances at 700 nm and 520 nm (A₇₀₀/A₅₂₀) linearly drops in the 5 to 100 ng·mL^?1 CLT concentration range, with a 3.6 ng·mL^?1 detection limit. This is below the Chinese guideline value for cucumber. The method is rather simple and does not require any modification of the AuNPs or the utilization of antibody. It was successfully applied to the determination of CLT in (spiked) cucumber samples. Recoveries ranged from 80.4 to 97.4%, and the analytical results compared well with those obtained by HPLC.

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Graphical abstract Schematic of the assay. The strong interaction of the cyano group of acetamiprid with gold nanoparticles (AuNPs) via Au-N bond induces the aggregation of gold nanoparticles, and this is accompanied by a color change from red to purple.

     

Colorimetric assay for cyanide and cyanogenic glycoside using polysorbate 40-stabilized gold nanoparticles

This study describes a simple method for the selective and sensitive detection of cyanide and endogenous biological cyanide using polysorbate 40-stabilized gold nanoparticles.     

Colorimetric adenosine aptasensor based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles

An aptamer based assay is described for the colorimetric detection of adenosine. The presence of adenosine triggers the deformation of hairpin DNA oligonucleotide (HP1) containing adenosine aptamer and then hybridizes another unlabeled hairpin DNA oligonucleotide (HP2). This leads to the formation of a double strand with a blunt 3′ terminal. After exonuclease III (Exo III)-assisted degradation, the guanine-rich strand (GRS) is released from HP2. Hence, the adenosine-HP1 complex is released to the solution where it can hybridize another HP2 and initiate many cycles of the digestion reaction with the assistance of Exo III. This leads to the generation of a large number of GRS strands after multiple cycles. The GRS stabilize the red AuNPs against aggregation in the presence of potassium ions. If, however, GRS forms a G-quadruplex, it loses its ability to protect gold nanoparticles (AuNPs) from salt-induced AuNP aggregation. Therefore, the color of the solution changes from red to blue which can be visually observed. This colorimetric assay has a 0.13 nM detection limit and a wide linear range that extends from 5 nM to 1 μM.

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Graphical abstract Schematic presentation of a colorimetric aptamer biosensor for adenosine detection based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles.

     

Colorimetric determination of ofloxacin using unmodified aptamers and the aggregation of gold nanoparticles

A colorimetric method is presented for the determination of the antibiotic ofloxacin (OFL) in aqueous solution. It is based on the use of an aptamer and gold nanoparticles (AuNPs). In the absence of OFL, the AuNPs are wrapped by the aptamer and maintain dispersed even at the high NaCl concentrations. The solution with colloidally dispersed AuNPs remains red and has an absorption peak at 520 nm. In the presence of OFL, it will bind to the aptamer which is then released from the AuNPs. Hence, AuNPs will aggregate in the salt solution, and color gradually turns to blue, with a new absorption peak at 650 nm. This convenient and specific colorimetric assay for OFL has a linear response in the 20 to 400 nM OFL concentration range and a 3.4 nM detection limit. The method has a large application potential for OFL detection in environmental and biological samples.

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Graphical abstract Schematic of a sensitive and simple colorimetric aptasensor for ofloxacin (OFL) detection in tap water and synthesic urine. The assay is based on the salt-induced aggregation of gold nanoparticles which results in a color change from red to purple.

     

Colorimetric assay for mercury (II) based on mercury-specific deoxyribonucleic acid-functionalized gold nanoparticles

A colorimetric nanoprobe-mercury-specific DNA-functionalized gold nanoparticles (Au-MSD) was developed for sensing Hg(2+). The new mercury-sensing concept relies on measuring changes in the inhibition of "non-crosslinking" aggregation of Au-MSD-induced by the folding of mercury-specific DNA strand through the thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination. In the absence of Hg(2+), a high concentration of MgCl(2) (50 mM) results in a rapid aggregation of Au-MSD because of the removal of charge repulsion. When Hg(2+) is present, the particles remain stable due to the folding of MSD functionalized on the particle surface. The assay enables the colorimetric detection of Hg(2+) in the concentration range of 0.1-10 μM Hg(2+) ions with a detection limit of 60 nM, and allows for the selective discrimination of Hg(2+) ions from the other competitive metal ions. Toward the goal for practical applications, the sensor was further evaluated by monitoring Hg(2+) in fish tissue samples.     

Colorimetric method for determination of bisphenol A based on aptamer-mediated aggregation of positively charged gold nanoparticles

A sensitive, specific and rapid colorimetric aptasensor for the determination of the plasticizer bisphenol A (BPA) was developed. It is based on the use of gold nanoparticles (AuNPs) that are positively charged due to the modification with cysteamine which is cationic at near-neutral pH values. If aptamers are added to such AuNPs, aggregation occurs due to electrostatic interactions between the negatively-charged aptamers and the positively-charged AuNPs. This results in a color change of the AuNPs from red to blue. If a sample containing BPA is added to the anti-BPA aptamers, the anti-BPA aptamers undergo folding via an induced-fit binding mechanism. This is accompanied by a conformational change, which prevents the aptamer-induced aggregation and color change of AuNPs. The effect was exploited to design a colorimetric assay for BPA. Under optimum conditions, the absorbance ratio of A ₅₂₇/A ₆₈₀ is linearly proportional to the BPA concentration in the range from 35 to 140 ng∙mL⁻¹, with a detection limit of 0.11 ng∙mL⁻¹. The method has been successfully applied to the determination of BPA in spiked tap water and gave recoveries between 91 and 106 %. Data were in full accordance with results obtained from HPLC. This assay is selective, easily performed, and in our perception represents a promising alternative to existing methods for rapid quantification of BPA.

The negatively-charged anti-BPA aptamers can absorb onto the positively-charged cysteamine-capped AuNPs (cysteamine-AuNPs) via electrostatic interactions, which can cause the aggregation of AuNPs accompanied by a red-to-blue color change. In the presence of BPA, the specific binding of BPA to the aptamers induces the conformation changes of anti-BPA aptamers, which can release the aptamers from cysteamine-AuNPs and thus prevent the aggregation and color change of cysteamine-AuNPs.

     

Sensing colorimetric approaches based on gold and silver nanoparticles aggregation: Chemical creativity behind the assay. A review

Localized surface plasmon resonance (LSPR) is one of the most remarkable features of gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs). Due to these inherent optical properties, colloidal solutions of Au and Ag NPs have high extinction coefficients and different colour in the visible region of the spectrum when they are well-spaced in comparison with when they are aggregated. Therefore, a well-designed chemical interaction between the analyte and NPs surroundings leads to a change of colour (red to blue for Au NPs and yellow to brown for Ag NPs from well-spaced to aggregated ones, respectively) allowing the visual detection of the target analyte.     

Colorimetric aptamer based assay for the determination of fluoroquinolones by triggering the reduction-catalyzing activity of gold nanoparticles

The authors describe a colorimetric assay for the detection of fluoroquinolones (FQs). It is based on the use of gold nanoparticles (AuNPs) modified with complementary DNA strands and analyte-specific FQ-binding aptamers. The modified AuNPs possess enzyme-like activity that can catalyze the reduction of nitrophenol by NaBH₄. In the absence of ciprofloxacin, the flower-shape coating on the AuNPs prevents the reduction of yellow 4-nitrophenol. In the presence of ciprofloxacin, the DNA/aptamer flower leaves on the AuNPs and the AuNPs can exert their catalytic activity. This results in a color change from yellow to colorless. The assay is highly selective for FQs, fast (1 h), and has a limit of detection as low as 1.2 nM in case of ciprofloxacin. It was successfully applied to the determination of ciprofloxacin in spiked water, serum and milk samples to give LODs of 1.3, 2.6 and 3.2 nM, respectively.     

A colorimetric sensor array for detection and discrimination of biothiols based on aggregation of gold nanoparticles

Developments of sensitive, rapid, and cheap systems for identification of a wide range of biomolecules have been recognized as a critical need in the biology field. Here, we introduce a simple colorimetric sensor array for detection of biological thiols, based on aggregation of three types of surface engineered gold nanoparticles (AuNPs). The low-molecular-weight biological thiols show high affinity to the surface of AuNPs; this causes replacement of AuNPs’ shells with thiol containing target molecules leading to the aggregation of the AuNPs through intermolecular electrostatic interaction or hydrogen-bonding. As a result of the predetermined aggregation, color and UV–vis spectra of AuNPs are changed. We employed the digital mapping approach to analyze the spectral variations with statistical and chemometric methods, including hierarchical cluster analysis (HCA) and principal component analysis (PCA). The proposed array could successfully differentiate biological molecules (e.g., cysteine, glutathione and glutathione disulfide) from other potential interferences such as amino acids in the concentration range of 10–800 μmol L⁻¹.     

Colorimetric assay for sulfate using positively-charged gold nanoparticles and its application for real-time monitoring of redox process

A flexible nanoparticle-based sulfate assay is demonstrated in which the positively-charged gold nanoparticles (cysteamine-AuNPs) act as indicator. The aggregation of cysteamine-AuNPs is selectively induced by sulfate, which allows the rapid colorimetric sensing of sulfate without any precipitant, sample preparation and specific instruments. In this work, the cysteamine-AuNPs probe has been successfully applied to the colorimetric detection of sulfate and demonstrates superior sensitivity with a detection limit of sulfate of ~50 ppb. A surprise finding is that the proposed probe can achieve the goal of real-time monitoring and translating a redox process into an appreciable color change via the aggregation of nanoparticles. This is a novel application of a positively-charged AuNPs-based nanoprobe for sulfate detection, kinetic study of the redox process, and opens up new opportunities for design of more novel colorimetric strategies and expansion of AuNPs-based application in different fields.     

A colorimetric assay for d-Penicillamine in urine and plasma samples based on the aggregation of gold nanoparticles

We report herein the development of a highly sensitive colorimetric method for detection of d-Penicillamine using citrate-capped gold nanoparticles (AuNPs). This assay relies upon the distance-dependent of gold nanoparticles surface plasmon resonance band of gold nanoparticles. By replacing the thiol-containing chelator drug, d-Penicillamine, with citrate on the gold nanoparticles surface, a new peak appearing at a longer wavelength intensifies and shifts further to the red from the original peak position due to aggregation of gold nanoparticles which depends on ionic strength, gold nanoparticles and d-Penicillamine concentration. During this process, the plasmon band at 521 nm decreases gradually along with the formation of a new red-shifted band at 630 nm. The calibration curve which is derived from the ratio intensities of absorbance at longer wavelength (630 nm) to original wavelength (521 nm) displays a linear relation in the range of 5.0 × 10^?6–3.0 × 10^?4 M d-Penicillamine. Lower limit of detection for d-Penicillamine, at the signal-to-noise ratio of 3 (3σ), was 3.8 × 10^?6 M. The developed methodology was successfully applied for the determination of d-Penicillamine in human urine and plasma.     

Simultaneous determination of aflatoxins, ochratoxin A, and zearalenone in grains by new immunoaffinity column/liquid chromatography

The simultaneous determination of mycotoxins was performed in 3 steps: extraction, cleanup, and detection. For extraction, a mixture of acetonitrile-water (60 + 40, v/v) was proved appropriate. For cleanup, a new Afla-Ochra-Zea immunoaffinity column was used. After derivatization with trifluoroacetic acid, the mycotoxins aflatoxins, ochratoxin A (OTA), and zearalenone (ZEA) were determined simultaneously by liquid chromatography with fluorescence detection. The detection limits in different matrixes after cleanup with the new immunoaffinity column were very low: aflatoxins, 0.002-0.7 microg/kg; OTA, 0.07-0.25 microg/kg; ZEA, 1-3 microg/kg. The limits of determination were: aflatoxins, 0.25 microg/kg; OTA, 0.5 microg/kg; ZEA, 5 microg/kg. The recovery rates for aflatoxins, OTA, and ZEA for rye and rice were between 86 and 93% when a 0.5 g sample matter per immunoaffinity column was used.     

Colorimetric and visual determination of total nereistoxin-related insecticides by exploiting a nereistoxin-driven aggregation of gold nanoparticles

Microchimica Acta - We describe a method for the visual and colorimetric determination of total nereistoxin-related insecticide residues. It is based on the nereistoxin-induced aggregation of gold...