Designing highly specific biosensing surfaces using aptamer monolayers on gold

To build highly specific surfaces using aptamer affinity reagents, the effects of linker and coadsorbents were investigated for maximizing target binding and specificity for aptamer-based self-assembled monolayers (SAMs) supported on gold. An aptamer that binds the protein thrombin was utilized as a model system to compare different mixed monolayer systems toward maximizing binding and selectivity to the immobilized aptamer. Important factors used to optimize binding characteristics of thrombin to the aptamer-based monolayer films include changes in design elements of the linker and different coadsorbent thiols. Binding events measured by surface plasmon resonance (SPR) and ellipsometry showed that the binding performance of the aptamer SAMs depends principally on the linker and to a lesser extent on the coadsorbent. SAMs formed with HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-aptamer exhibited a 4-fold increase in binding capacity versus SAMs made using HS-(CH2)6-TTTTT-aptamer. Furthermore, SAMs made using HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-aptamer showed nearly complete specificity for thrombin versus bovine serum albumin (BSA, less than 2% bound), while a SAM incorporating a random DNA fragment (HS-(CH2)6-OP(O)2O-(CH2CH2O)6-TTTTT-RANDOM) showed little binding of thrombin. Irrespective of the aptamer-linker system, use of HS-(CH2)11(OCH2CH2)3OH, referred to as EG(3), as a coadsorbent enhanced binding of thrombin by approximately 2.5-fold compared to that of HS-(CH2)6-OH (mercaptohexanol, MCH).     

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 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.     

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 multiplexed immunoassay using specific aggregation of antigenic peptide-modified luminous nanoparticles

Two amperometric enzyme biosensor systems, based on glycerol dehydrogenase/diaphorase (GDH/DP) and glycerol kinase/glycerol-3-phosphate oxidase/peroxidase (GK/GPOx/HRP), were developed and used for estimation of glycerol content in a complex biological fluids. Enzymes were immobilized on interchangeable membranes by PCS-prepolymer technique. Buffers containing ferricyanide/NAD⁺ or ferrocyanide/ATP were used for measurements with GDH/DP and GK/GPOx/HRP biosensor, respectively. FIA assay of glycerol biosensor was characterized by a linear range of 0.01-1 or 0.01-1.5 mM glycerol, sensitivity of 6.02 or 1.42 mA/M cm² and with signal loss of 40% after 90 h or 30% after 16 h during continuous operation at a sample throughput of 10 injections/h for GDH/DP or GK/GPOx/HRP biosensors, respectively. Both biosensors were successfully used for off-line monitoring of glycerol during microbial transformation of glycerol to 1,3-propanediol using an automatized flow-through system. The results were consistent with those obtained with HPLC. The stability of described biosensor systems was sufficient for monitoring and control of fermentation process within 24 h. The storage stability of enzyme membranes was several months.     

Colorimetric kinetic determination of potassium ions based on the use of a specific aptamer and catalytically active gold nanoparticles

We describe a simple, highly sensitive, and selective colorimetric kinetic assay for the determination of potassium(I) by exploiting the specific recognition capability of an appropriate aptamer and catalytic signal amplification by gold nanoparticles (AuNPs). Amplification is based on the reduction of 4-nitrophenol by borohydride which is catalyzed by AuNPs. This leads to a color change of the solution from yellow to colorless, and the color change can be recognized with bare eyes or via photometry. The K(I)-selective aptamer is placed on the AuNPs and forms a tightly bound G-quadruplex with K(I) which partially masks the surface of the AuNPs and prevents 4-nitrophenol to be reduced at the catalytically active surface of the AuNPs. Hence, the rate of decoloration is retarded. The assay displays high selectivity for K(I) over other cations, has a linear response in the 0.1 nM to 10 μM concentration range, and a detection limit as low as 0.06 nM. In addition, these findings pave the way to novel analytical methods based on the use of gold nanoparticle-catalyzed chemical reactions.

A simple, highly sensitive, and selective colorimetric kinetic assay for the determination of potassium(I) was represented.

     

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 sensing of anions in water using ratiometric indicator-displacement assay

The analysis of anions in water presents a difficult challenge due to their low charge-to-radius ratio, and the ability to discriminate among similar anions often remains problematic. The use of a 3 × 6 ratiometric indicator-displacement assay (RIDA) array for the colorimetric detection and identification of ten anions in water is reported. The sensor array consists of different combinations of colorimetric indicators and metal cations. The colorimetric indicators chelate with metal cations, forming the color changes. Upon the addition of anions, anions compete with the indicator ligands according to solubility product constants (K_sp). The indicator–metal chelate compound changes color back dramatically when the competition of anions wins. The color changes of the RIDA array were used as a digital representation of the array response and analyzed with standard statistical methods, including principal component analysis and hierarchical clustering analysis. No confusion or errors in classification by hierarchical clustering analysis were observed in 44 trials. The limit of detection was calculated approximately, and most limits of detections of anions are well below μM level using our RIDA array. The pH effect, temperature influence, interfering anions were also investigated, and the RIDA array shows the feasibility of real sample testing.     

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.     

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 detection of potassium ions using aptamer-functionalized gold nanoparticles

Herein, a simple and novel colorimetric method for detection of potassium ions (K⁺) was developed. The colorimetric experiments revealed that upon the addition of K⁺, the conformation of anti-K⁺ aptamer in solution changed from random coil structure to compact rigid G-quadruplex one. This compact rigid G-quadruplex structure could not protect AuNPs against K⁺-induced aggregation, and thus the visible color change from wine-red to blue-purple could be observed by the naked eye. The linear range of the colorimetric aptasensor covered a large variation of K⁺ concentration from 5 nM to 1 μM and the detection limit of 5 nM was obtained. Moreover, this assay was able to detect K⁺ with high selectivity and had great potential applications.     

Colorimetric detection of thiol-containing amino acids using gold nanoparticles

This paper reports findings of an investigation of the unusual colorimetric change of gold nanoparticles in the presence of thiol-containing amino acids such as homocysteine, cysteine and glutathione. The colorimetric change for homocysteine exhibits a rate that is about two orders of magnitude higher than that for cysteine, and at least five orders of magnitude higher than that for glutathione. The reactivity is effectively reduced or suppressed by the coexistence of either cysteine or glutathione. It is believed that the reactivity involves encapsulation of the particles by the thiol-containing amino acids which is followed by crosslinking at the encapsulating shells. In comparison with cysteine and glutathione, homocysteine has a slower encapsulating rate but a faster crosslinking rate. Implications of the findings of the interfacial encapsulation and crosslinking reactivities of gold nanoparticles to potential nanoparticle-enhanced analytical detection of thiol-containing amino acids are also briefly discussed.     

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...     

Colorimetric detection of c-Kit mutations using electrostatic attraction induced aggregation of peptide nucleic acid modified gold nanoparticles

We report a colorimetric detection of c-Kit mutations using selective aggregation of the peptide nucleic acid modified gold nanoparticles that is caused by electrostatic attraction.