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This report describes dual-channel colorimetric array sensors for rapid distinction of metal ions. This dual-channel sensing array is built based upon gold nanoparticles (AuNPs)-catalyzed two colorimetric reactions [3,3′,5,5′-tetramethylbenzidine (TMB) and H2O2 as channel 1 and HAuCl4 and NH2OH as channel 2]. Different metal ions show diverse interactions toward AuNPs, producing differential variations of AuNPs' catalytic ability and yielding distinct colorimetric response modes (channel 1). Different metal ions show different affinities toward the DNA absorbed on the AuNP surface, and the decrease of the amount of DNA entangled on the AuNPs changes the catalytic ability of the AuNPs to varying degrees, generating varying visual response modes (channel 2). These signal modes as "fingerprints" are typical for each metal ion and are analyzed by linear discriminant analysis (LDA). The results display clear discrimination of each analyte on a LDA plot, and 10 kinds of metal ions are well identified at a low concentration (20 nM). [ABSTRACT FROM AUTHOR]

An effective and robust colorimetric sensor array for simultaneous detection and discrimination of five reducing sugars (i.e., glyceraldehyde (Gly), fructose (Fru), glucose (Glu), maltose (Mal), and ribose (Rib)) has been proposed. In the sensor array, two negatively charged polydielectrics (sodium polystyrenesulfonate (NaPSS) and sodium polymethacrylate (NaPMAA)), which served as the sensing elements, were individually absorbed on the surface of the cetyltrimethylammonium bromide (CTAB)-coated gold nanorods (AuNR) with positive charges through electrostatic action, forming the designed sensor units (NaPSS-AuNR and NaPMAA-AuNR). In the presence of Tollens reagent (Ag(NH3)2OH), Ag+ was absorbed on the surface of negatively charged NaPSS-AuNR and NaPMAA-AuNRs. When confronted with differential reducing sugars, different reducing sugars exhibited differential levels of deoxidizing abilities toward Ag+, thus Ag+ was reduced to diverse amounts of silver nanoparticles (AgNPs) in situ to form core-shell AuNR@AgNP by the traditional Tollens reaction method, leading to distinct colorimetric response patterns (value of AS/AL (the ratio of absorbance at 360 nm to that at 760 nm in Ag+-NaPMAA-AuNR, and the ratio of absorbance at 360 nm to that at 740 nm in Ag+-NaPSS-AuNR)). These response patterns are characteristic for each reducing sugar, and can be quantitatively distinguished by linear discriminant analysis (LDA) at concentrations as low as 10 nM with relative standard deviation (RSD) of 4.11% (n = 3). The practicability of this sensor array has been validated by recognition of reducing sugars in serum and urine samples. [ABSTRACT FROM AUTHOR]

A colorimetric method is described for the determination of uric acid (UA). The assay is based on oxidative etching of silver nanoparticles (AgNPs) by chloroauric acid (HAuCl4). The presence of UA suppresses the redox reaction between AgNPs and HAuCl4 because a competitive reaction occurs between HAuCl4 and UA. This results in a color change of the solution from brown to yellow. In parallel, the absorbance is blue shifted from 477 to 428 nm. The method has a detection limit as low as 30 pM (at S/N = 3) and a linear response range that covers the 0.1 nM to 0.1 mM UA concentration range. The reliability of the method was successfully demonstrated by analyzing spiked serum samples. [ABSTRACT FROM AUTHOR]

A colorimetric array for metal ions is described that is based on the de-aggregation of papain-capped gold nanoparticles (AuNPs) for discrimination of metal ions. The functionalized AuNPs were used as colorimetric probes, and three chelating agents (tripolyphosphate, citrate, and ethylenediamine tetraacetate were employed as the recognition receptors. In the absence of metal ions (Fe3+, Cr3+, Co2+, Mg2+, Pb2+, Ca2+, Zn2+, Ti4+, and Sn4+ were studied), the capped AuNPs are well dispersed in solution. In the presence of these ions, the metal ions bind papain and cause the AuNPs to aggregate. This causes a color change from red to purple. Different chelating agents induce different affinities between metal ions and papain. This results in distinct colorimetric response patterns. Linear discriminant analysis is used to discriminate the various metal ions on a three-dimensional spatial dispersion graph. [ABSTRACT FROM AUTHOR]

There is growing interest in developing a high-performance sensor array for detection and discrimination of antioxidants owing to their widespread use and essential role in the human body. The present work unveils a novel colorimetric sensor array for colorimetric discrimination of antioxidants based on the red, green, and blue alteration (ΔRGB) pattern recognition. In this sensor array, three concentrations of AgNO3 were used as sensing elements, and gold nanoparticles (AuNPs) were employed as a colorimetric probe. In the presence of antioxidants, the sensor array produces unique colorimetric response patterns for the discrimination of these antioxidants due to different reactivities between three different concentrations of AgNO3 and each antioxidant, leading to deposition of different quantities of Ag nanoshells on the surface of AuNPs, enabling an excellent discrimination of six antioxidants (catechin, epigallocatechin 3-gallate, epicatechin, epigallocatechin, epicatechin 3-gallate, and gallocatechin) at a 20 nM level, when linear discriminant analysis (LDA), hierarchical cluster analysis (HCA), centroid diagram, spidergram, and color contour profiles were smartly combined. Furthermore, different concentrations of antioxidants and binary antioxidant mixtures, even ternary mixtures, could also be discriminated with this sensor array. Finally, the sensor array was successfully used for the discrimination of antioxidants in serum samples, demonstrating its potential applications in the diagnosis of antioxidant-related diseases. [ABSTRACT FROM AUTHOR]

Proteins play a key role in disease diagnosis, and protein discrimination is an important but difficult issue. Here, we report a novel strategy for improving protein discrimination through a facile colorimetric sensor array, which is based on DNA–gold nanoparticle (AuNP) conjugates manipulated by exonuclease I (Exo I). Different proteins exhibit diverse affinities toward the three DNAs, and the DNA–protein binding is resistant to the digestion of Exo I and protects the AuNPs from aggregation in high concentrations of NaCl media, forming distinct response patterns of the array. These response patterns as “fingerprints” can be acquired on the sensor array and then identified by linear discriminant analysis (LDA). The sensor array achieved the correct discrimination of 15 proteins at a 10 nM level in buffer solution and real serum samples. Also, the sensor array had the capability to discriminate individual proteins and the mixtures of them. Remarkably, the practicability of the sensor array was further confirmed by the identification of 35 unknown protein samples with 100% accuracy. [ABSTRACT FROM AUTHOR]

We present an ultrasensitive colorimetric nucleic acid assay based on the intensity of gold nanoparticles (Au NPs) using dark field microscopy. In the absence of target DNA, two hairpin-like DNA strands with protruding single-stranded DNA (ssDNA) can be absorbed onto the Au NP surface via non-covalent interactions between the exposed nitrogen bases of ssDNA and Au NPs, which inhibits Au NPs from aggregating in a high concentration of salt media, while in the presence of target DNA, two hairpin DNA strands hybridize with target DNA to form double-stranded DNA (dsDNA). After hybridization chain reaction (HCR) amplification, rigid dsDNA polymers are formed, which results in serious Au NP aggregation in the salt environment. By measuring the intensity change of yellow and red dots on a dark-field image, the concentration of target DNA can be accurately quantified with a limit of detection (LOD) of 66 fM. [ABSTRACT FROM AUTHOR]

Under normal circumstances, chromate ions cannot oxidize core-shell Cu-Au nanoparticles (NPs) because the standard redox potential of the Cr(VI)/Cr(III) system is lower than that of Au(I)/Au. However, in the presence of a certain concentration of hydrogen bromide, the Nernst potential becomes higher than that of Au(I)/Au and Cu(II)/Cu (as calculated by the Nernst equation). Based on this finding, a colorimetric method was developed that is based on oxidative etching of Cu@Au NPs by chromate. The Cu-Au NPs have a light red color in the presence of 5 mM HBr. On addition of Cr(VI), the gold shell is rapidly etched from the surface of the Cu-Au NPs, and this is accompanied by a color change to blue. The color changes were characterized by UV-vis spectroscopy. The absorbance at 526 nm changes in the 0.4 nM to 1 μM chromate concentration range, and the detection limit is 0.21 nM. [ABSTRACT FROM AUTHOR]

A series of single-strand oligonucleotides functionalized catalytically active gold nanoparticle (AuNPs) as nonspecific receptors have been designed to build a protein sensing array. We take advantage of the correlation between the catalytic activity and the exposed surface area of AuNPs, i.e., DNA-proteins interactions mask the surface area of AuNPs, leading to poor catalytic performance of AuNPs. As the number of DNA-bound proteins increases, the surfaces of AuNPs become more masked; thus, the time of 4- nitrophenol/NaBH4 reaction for color change (yellow ? colorless) of the solution increases. Taking advantage of three nonspecific SH-labeled DNA sequences (A15, C15, and T15) as array sensing elements and the color-change time (CCT) of the solution as signal readout, colorimetric response patterns can be obtained on the array and identified via linear discriminant analysis (LDA). Eleven proteins have been completely distinguished with 100% accuracy with the naked eye at the 30 nM level. Remarkably, two similar proteins (bovine serum albumin and human serum albumin), two different proteins (bovine serum albumin and concanavalin) at the same concentration, and the mixtures of the two proteins with different molar ratios have been discriminated with 100%. The practicability of this sensor array is further validated by high accuracy (100%) identification of 11 proteins in human serum samples. [ABSTRACT FROM AUTHOR]

We report on a method for highly sensitive and selective colorimetric determination of Hg(II) via a signal amplification strategy. Cu@Au nanoparticles (NPs) are found to exhibit intrinsic peroxidase-like activity and can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine by HO. This is accompanied by a solution color change from colorless to green (with an absorption peak at 655 nm). The catalytic capability of the Cu@Au NPs (pale green) is strongly enhanced by a Cu@Au-Hg trimetallic amalgam (bluish), and this effect can be applied directly to the determination of Hg(II). The limit of detection as observed with the unaided eye is 10 nM, which is at least one order of magnitude lower than that of the known AuNP-based colorimetric assay. Due to excellent specificity of the amalgamation process, the assay is highly selective for Hg(II) and is not interfered by other metal ions in up to 0.5 μM concentrations. This assay was successfully applied to the determination of Hg(II) in tap water. In view of these advantages, we expect this colorimetric method to become an attractive tool for the quantitation of Hg(II) in biological, environmental, and food samples. [ABSTRACT FROM AUTHOR]

The authors have developed a straightforward colorimetric method for the rapid determination of lysozyme by using citrate-capped gold nanoparticles (AuNPs) with different particle sizes but without any further surface modification. It is found that AuNPs (15 nm i.d.) undergo aggregation in the presence of lysozyme owing to the high abundance of amino groups in lysozyme. Aggregation leads to a color change of the solution from red over purple to bluish-purple that can be detected visually or by photometry. The limit of detection is 20 nM. We further show that the use of AuNPs with 5 and 15 nm i.d. can improve the sensitivity of the assay compared to using bare AuNPs by adding HAuCl and NHOH to the solution which induces the growth of AuNPs and leads to a smaller interparticle space between AuNPs. This gives rise to differently colored solutions, with color transitions from red to bluish-purple to colorless. The LODs are 0.1 nM for both the 5-nm and 15-nm AuNPs. Compared to the LOD when using a solution of 15-nm AuNPs and without chloroauric acid and hydroxylamine, the LOD (0.1 nM) is lower by a factor of 200. The method is sensitive, specific, and does not require sophisticated equipment. Its feasibility was demonstrated by analyzing lysozyme in samples of egg white. [ABSTRACT FROM AUTHOR]

We have presented an extensible, facile and sensitive multidimensional sensor based on DNA-gold nanoparticle (DNA-AuNP) conjugates for heavy metal ions (Ag + , Hg 2+ , Cr 3+ , Sn 4+ , Cd 2+ , Cu 2+ , Pb 2+ , Zn 2+ , and Mn 2+ ) discrimination. In the presence of metal ions, the excluded effect of DNA and AuNPs with the same negative charges is disrupted, and the amount of FAM-labeled DNA adsorbed on AuNP surfaces increases, resulting in a more obvious fluorescence quenching effect. With the addition of NH 2 OH and HAuCl 4 , AuNPs grow into morphologically varied nanostructures (spherical to branched) depending on the resulting aptamer coverage, which gives rise to different colored solutions (reddish blush, purple and blue) observed by naked eyes. By simply changing the DNA sequences, three sensing elements can be easily obtained and added into this dual-channel multidimensional sensor. 9 heavy metal ions are distinguished by linear discriminant analysis (LDA) and primary component analysis (PCA). A highly sensitive discrimination of metal ion targets with the detection limit as low as 50 nM with 100% identification accuracy is obtained. Remarkably, Cu 2+ and Hg 2+ ions with similar catalytic performance at various concentrations (300 nM, 400 nM, 500 nM, respectively) and the mixture of the two metal ions with different volume ratios (total metal ion concentration: 500 nM) can be successfully discriminated. In addition, nine heavy metal ions are also well-distinguished in river samples, and the accuracy of discrimination of these metal ions samples reaches 100%. Therefore, it will broaden the application field of DNA-AuNP conjugates-based multidimensional sensors. [ABSTRACT FROM AUTHOR]

In this work, we report a simple, ultrasensitive, and feasible colorimetric assay for metal ion (K + , used as a model) via inherent peroxidase-like enzymatic amplification strategy of gold nanoparticles (AuNPs). It is shown that peroxidase-like activity of AuNPs can be improved dramatically by its surface activation with target-specific aptamer molecules. Whereas when the target exists, the aptamers leave the surface of AuNPs in a target concentration-dependent manner, resulting in a decrease of the nanoenzymatic catalytic ability of AuNPs. Thus, K + can be quantified in the presence of AuNPs by using a colorimetric sensing probe (3,3′,5,5′-tetramethylbenzidine). The color change of the solution is relevant to the dose of the target, and this can be achieved with the naked eyes and monitored by UV–vis spectrometry. A linear dependence between the absorbance and target K + concentration is obtained under optimal conditions in the range from 0. 1 nM to 1 μM with a detection limit (LOD) of 0.06 nM estimated at the 3S blank level . The sensitivity displays to be 2–9 orders of magnitude better than those of other K + detection methods. This sensing strategy may in principle be universally applicable for the detection of a range of environmental or biomedical molecules of interest. [ABSTRACT FROM AUTHOR]

Herein, a label-free and highly sensitive electrochemical aptasensor for the detection of angiogenin was proposed based on a conformational change of aptamer and amplification by poly(diallyldimethyl ammonium chloride) (PDDA)-functionalized graphene/gold nanoparticles (AuNPs) composites-modified electrode. PDDA-functionalized graphene (P-GR) nanosheets as the building block in the self-assembly of GR nanosheets/AuNPs heterostructure enhanced the electrochemical detection performance. The electrochemical aptasensor has an extraordinarily sensitive response to angiogenin in a linear range from 0.1 pM to 5 nM with a detection limit of 0.064 pM. The developed sensor provides a promising strategy for the cancer diagnosis in medical application in the future. [ABSTRACT FROM AUTHOR]

We provide a highly sensitive and selective colorimetric assay to detect mercury ions (Hg 2+ ) in aqueous environment using thiocyanuric acid (TCA) molecule-functionalized gold nanoparticles (AuNPs). This method is based on the thiophilicity of Hg 2+ and AuNPs as well as the unique optical properties of TCA-functionalized AuNPs. In the presence of TCA, AuNPs aggregate due to the strong attraction between thiol groups of TCA and surface-bounded AuCl 4 - /AuCl 2 - ions, which induces the visible color change from red to blue. With the addition of Hg 2+ , Hg 2+ is more apt to interact with thiols than AuNPs. Thus, Hg 2+ can remove the AuNPs of the TCA-functionalized AuNPs and trigger AuNP aggregation redisperse again. This assay can selectively detect Hg 2+ with the detection limits as low as 0.5 nM in aqueous solution. [ABSTRACT FROM AUTHOR]

Abstract: A colorimetric assay for the ultrasensitive determination of thrombin based on cationic polymer and gold nanoparticles was presented, in which unmodified gold nanoparticles (AuNPs) was used as probes and 21-mer thrombin-binding aptamer (TBA) as sensing elements. Upon the addition of thrombin, TBA interacted specifically with thrombin to form a G-quadruplex structure. As a result, the conformation change facilitated the cationic polymer, poly(diallyldimethylammonium chloride) (PDDA)-induced AuNP aggregation. Thus, the visible change in color from wine-red to blue–purple was readily seen by the naked eye. The colorimetric sensor could detect thrombin down to 1pM with high selectivity in the presence of other interferring proteins. Furthermore, the assay was successfully employed to determine thrombin in human serum sample, which suggested its great potential for diagnostic purposes. [Copyright &y& Elsevier]

Highlights: [•] A simple and novel colorimetric method for detection of K+ was developed. [•] The aptasensor exhibited low detection limit and wide linear range to K+. [•] This method was able to detect K+ with high selectivity. [•] This colorimetric method exhibited excellent applicability. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we described signal amplification architecture for electronic aptamer-based sensor (E-AB), which is applicable to a wide range of aptamers. Herein, we only take lysozyme as the representative sensing target. The amplification method was based on the network of thiocyanuric acid (TCA)/gold nanoparticles (AuNPs) modified with ssDNA. The binding event can be detected by a decrease in the integrated charge of the surface-bound [Ru(NH3)6]3+ which electrostatically absorbed onto the negatively charged phosphate backbones of DNA. In the presence of target molecules, a large amount of TCA/AuNP/ssDNA network associated with [Ru(NH3)6]3+ would be removed from the electrode surface, leading to a significant decrease of redox current. Cyclic voltammetry (CV) signals of [Ru(NH3)6]3+ provides quantitative measures of the concentrations of lysozyme, with a linear calibration ranging from 5pM to 1nM and a detection limit is 0.1pM. The detection limit of the proposed sensor is one order of magnitude and three orders of magnitude more sensitive than the detection limits in the absence of TCA (5pM) and in the absence of TCA/AuNP/ssDNA network (0.5nM). This amplification method is promising for broad potential application in clinic assay and various protein analysis. [Copyright &y& Elsevier]

Abstract: A highly sensitive and specific electrochemical aptasensor for Cu2+ detection based on gold nanoparticles (AuNPs) is presented. In this work, AuNPs offered a big surface area to immobilize a large number of aptamers and excellent electrochemical signal transduction. Its high sensitivity, low detection limit, and wide detection range are the main advantages over our former copper aptasensor. The peak current increased proportionally to the Cu2+ concentration over the range from 0.1nM to 10μM with a detection limit of 0.1pM. The presence of other divalent metal ions did not affect the detection of Cu2+, which indicates a high specificity of Cu2+ detection could be detected. Rapidity, simplicity, and excellent selectivity make it suitable for practical use in determination of Cu2+ from lake samples. [Copyright &y& Elsevier]

Abstract: A simple, highly sensitive, and label-free electrochemical impedance spectroscopy (EIS) aptasensor based on an anti-lysozyme-aptamer as a molecular recognition element, was developed for the detection of lysozyme. Improvement in sensitivity was achieved by utilizing gold nanoparticles (AuNPs), which were electrodeposited onto the surface of a gold electrode, as a platform for immobilization of the aptamer. To quantify the amount of lysozyme, changes in the interfacial electron transfer resistance (R et) of the aptasensor were monitored using the redox couple of an [Fe(CN)6]3−/4− probe. The R et increased with lysozyme concentration. The plot of R et against the logarithm of lysozyme concentration is linear over the range from 0.1pM to 500pM with a detection limit of 0.01pM. The aptasensor also showed good selectivity for lysozyme without being affected by the presence of other proteins. [ABSTRACT FROM AUTHOR]

An optical method for determination of Hg(II) is described that exploits the aggregation of gold nanoparticles (AuNPs) under dark-field microscope (DFM) observation. This assay is based on the use of a Hg(II)-specific aptamer, AuNPs modified with complementary DNA strands, and exonuclease I (Exo I). In the absence of Hg(II), the added dsDNA prevents salt-induced aggregation of the green-colored AuNPs. If Hg(II) is added, the aptamer will capture it to form T-Hg(II)-T pairs, and the complementary strand is digested by Exo I. On addition of a solution of NaCl, the AuNPs will aggregate. This is accompanied by a color change from green to orange/red) in the dark-field image. By calculating the intensity of the orange/red dots in the dark-field image, concentration of Hg(II) can be accurately determined. The limit of detection is as low as 36 fM, and response is a linear in the 83 fM to 8.3 μM Hg(II) concentration range. [ABSTRACT FROM AUTHOR]

A colorimetric method is presented for ultrasensitive determination of adenosine. The assay is based on side-by-side self-assembly of aptamer-functionalized gold nanorods (Au NRs). It relies on the fact that the conjugation of the helper DNA predominantly occurs at the terminal ends of the Au NRs rather than at their sides. The adenosine aptamers consist of two pieces of ssDNA (termed C1 and C2) that were individually attached to the sides of Au NRs. In the presence of adenosine, it will be captured by C1 and C2 to form a stable sandwich structure. As a result, a side-to-side assembly of the Au NRs occurs. If the adenosine concentration is increased, the absorbance of the Au NRs at 742 nm gradually decreases, and the color changes from brick red to dark brown. Response is linear range in the 10 pM to 5 nM adenosine concentration range, and the detection limit is as low as 3.3 pM. Adenosine analogues such as uridine and cytidine do not interfere. The method was used to quantify adenosine in serum samples at concentrations as low as 10 pM. [ABSTRACT FROM AUTHOR]

A colorimetric approach is presented for the determinaton of cadmium(II) using unmodified gold nanoparticles (AuNPs) as a colorimetric probe. The assay is based on AuNP aggregation that is induced by the complexes formed between Cd(II) and glutathione (GSH) in a concentrated solution of NaCl. Unmodified AuNPs are known to aggregate in high-salt medium, but GSH can prevent aggregation. In the presence of Cd(II), it will bind GSH, and this will cause the AuNPs to aggregate as indicated by yellow and red dots under dark-field microscopy observation and the formation of a blue coloration. By monitoring the intensity change of AuNPs (as a ratio of absorbances at 600 and 520 nm), Cd(II) can be quantified with a linear response in the 17 pM to 16.7 nM concentration range and a detection limit of 4.3 pM. The method was successfully applied to the determination of Cd(II) in spiked lake water by the standard addition mode, and the detection limit is 4.5 pM.A ultrasensitive colorimetric assay of cadmium ions using unmodified gold nanoparticles as colorimetric probes with dark-field microscopy [ABSTRACT FROM AUTHOR]

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.Schematic presentation of a colorimetric aptamer biosensor for adenosine detection based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles. [ABSTRACT FROM AUTHOR]

A rapid, highly sensitive and selective colorimetric assay is presented for visually detecting L-histidine. It is based on L-histidine-triggered self-cleavage of DNA duplex-induced gold nanoparticle (AuNP) aggregation. The citrate-capped AuNPs easily aggregate in a high concentration of salt environment. However, in the presence of L-histidine aptamers (DNA1 and DNA2), the partial strands of DNA1 and DNA2 hybridize to form a DNA duplex with a swing structure. The swing-like DNA duplexes are adsorbed on the surface of AuNPs to improve the stability of AuNPs, and the AuNPs also are better dispersed in high-salt media. When L-histidine is added to the solutions, it catalyzes the self-cleavage of DNA1 to form many single-stranded DNA (ssDNA) fragments. These ssDNA segments are adsorbed on the AuNPs and weaken the stability of AuNPs. Hence, the AuNPs aggregate in high-salt environment, and this results in a red-to-blue color change. Under the optimized conditions, L-histidine can be determined with a limit of detection of 3.6 nM. In addition, the sensor was successfully applied to the determination of L-histidine in spiked serum samples.Schematic of a rapid and homogeneous colorimetric L-histidine assay. It combines L-histidine-triggered self-cleavage of the swing-like DNA duplexes and self-cleavage of DNA-induced AuNP aggregation. [ABSTRACT FROM AUTHOR]

The authors describe a colorimetric method for the determination of DNA based on the deaggregation of gold nanoparticles (AuNPs) induced by exonuclease III (Exo III). DNA amplification is accomplished by Exo III to generate large quantities of the residual DNA. Residual DNA tethers onto the surfaces of AuNPs which prevents their aggregation. Hence, the color of the solution is red. However, in the absence of DNA, salt-induced aggregation is not prevented, and the bluish-purple color of the aggregated AuNPs is observed. The ratio of absorbances at 525 and 625 nm increases up to 150 nM DNA concentrations, and the LOD is as low as 3.0 nM. It is shown that the presence of 300 nM concentrations of random DNA (with a mass up to 10-fold that of target DNA) does not interfere. The method was successfully applied to the analysis of DNA in spiked serum samples. The method is simple, reliable, and does not require complicated amplification steps and expensive instrumentation.Schematic of a sensing strategy for DNA detection by exonuclease III-induced deaggregation of gold nanoparticles. DNA concentrations as  low as 3 nM can be detected via colorimetric monitoring of the color change from red to purple-blue. [ABSTRACT FROM AUTHOR]

We have proposed a colorimetric sensing assay exploiting the formation of gold amalgamation, combining with Hg 2+ mediated the growth of gold nanoparticles (AuNPs) for rapid detecting Hg 2+ ions with highly sensitivity and selectivity. In our developed approach, Hg 2+ is reduced to elemental mercury by citrate-capped AuNPs, forming gold amalgamation on the surface of AuNPs. AuNP-Hg 2+ interaction modulates the growth of AuNPs which is formed in the HAuCl 4 /NH 2 OH reaction. The rate of AuNP growth is highly dependent on the target concentration. AuNPs with low Hg coverage grow into spherical AuNPs with slow growth rate, which generates red-colored solutions. Whereas, AuNPs with high Hg coverage accelerate the growth of spherical AuNPs with a larger particle size, which generates blue colored solutions. At higher concentration, however, the growth rate of AuNPs begins to decrease. As the concentration of Hg 2+ increases till the saturation of Hg onto the surface of AuNPs, the growth of the AuNPs is inhibited due to full coverage of Hg on the AuNP surface. On the basis of this principle, Hg 2+ can be well determined ranging from 0.5 to 10 nM with a 0.26 nM of the detection limit. [ABSTRACT FROM AUTHOR]

A growing body of evidence indicates that lysozyme plays a significant role as an indicator for many diseases and a drug for treatment of infections, ulcers and to study the spatial conformation, enzyme kinetics, and molecular immunology. Therefore, highly sensitive determination of lysozyme is necessary and vital in a wide variety of fields. In this work, we put forward a simple but effective strategy for colorimetric visualization of lysozyme based on iodide-responsive Cu@Au nanoparticles (Cu@Au NPs) as well as the iodide-catalyzed H 2 O 2 -TMB (3,3,5,5-tetramethylbenzidine) reaction system. Colorimetric detection is applied because of its simplicity, fast response for analysis, high detection limit, low costs and practicality. In our strategy, iodide is applied for the reason that it can induce an obvious color change of the Cu@Au nanoparticles solution from gray to red, along with the change of morphologies of the Cu@Au nanoparticles from irregular to spherical. Consequently, this phenomenon results in colorimetric signal variation of the iodide-catalytic H 2 O 2 -TMB system. What's more, by quite simple biomolecule modification on the Cu@Au nanoparticles surface, an all-purpose colorimetric platform is established for the accurate detection of lysozyme, which could lead to the change of Cu@Au NP concentration through molecular recognition. The results show that modified Cu-Au NPs successfully achieved a simple, selective, visualized, and ultrasensitive detection of lysozyme with a linear range from 10 −7 to 10 −3 M and a detection limit of 60 nM. [ABSTRACT FROM AUTHOR]

We present a simple and efficient colorimetric assay strategy for ultrasensitive visual detection of human α-thrombin, which is essentially based on the formation of the DNA1-thrombin-DNA2 sandwich complex-bridged gold nanoparticle (Au NP) oligomers. Unlike the traditional colorimetric sensing strategies which induced the nanoparticle aggregates with uncontrolled aggregate size. In this work, the DNA1with rich G bases was firstly conjugated on the surfaces of Au NPs fixed on the hexadecyl trimethylammonium bromide (CTAB)-coated glass slide, and thrombin was captured by the DNA1. Then, the other DNA2 with rich G bases interacted with the former DNA1-thrombin complex and formed a DNA1-thrombin-DNA2 sandwich complex. The subsequently added Au NPs can be bound to the Au NP-DNA1-thrombin-DNA2 via Au-S bond to trigger the formation of Au NP oligomers, an apparent color change of the single Au NPs from green to yellow and red was observed under dark field microscopy. By measuring the intensity change of the yellow and red Au NPs, the concentration of target thrombin could be accurately quantified. As a proof of concept experiment, the formation of Au NP oligomers resulted in significantly improved sensitivity (10 fM of limit of detection and 20 fM of limit of quantity) and wider linear dynamic range of thrombin detection (20 fM-20 nM), the relative standard deviation (RSD) was less than 5.73% (n = 5). In addition, in order to validate the potential application in clinical diagnosis, the content of thrombin in a human serum samples was also quantified. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultrasensitive detection of adenosine is realized based on three-dimensional Au/TiO 2 nanorod arrays. • The TiO 2 nanorod arrays grown on FTO substrate can attach a large number of Au nanoparticles. • Among these five Au/TiO 2 nanorod arrays, the sensing performance of Au/TiO 2 -(0.65)-nanorod array is the best. • The aptasensor showed good accuracy, excellent selectivity, superb stability, and good application in serum. An electrochemical aptasensor was constructed based on Au/TiO 2 nanorod arrays for ultrasensitive detection of adenosine. Specifically, The TiO 2 nanorod arrays grown on FTO substrate can attach a large number of Au nanoparticles (AuNPs), which greatly increase the immobilization of aptamers and significantly improve the redox signal of electrostatically adsorbed [Ru(NH 3) 6 ]3+. In the presence of adenosine, the number of [Ru(NH 3) 6 ]3+ electrostatically adsorbed on the aptamer skeleton decreases, resulting in a decline in the detected square wave voltammetry (SWV) signal. Under the optimal conditions, the peak SWV current values of aptasensor based on five TiO 2 nanorod arrays are linear with the logarithmic concentration of adenosine, and the minimum detection limit is 0.42 fM. More importantly, the aptasensor has shown high specificity, extraordinary precision, continuous stability, as well as wonderful applicability for adenosine. Hence, the aptasensor paves the way for developing an effective clinical diagnostic means for adenosine detection. [ABSTRACT FROM AUTHOR]

We present here a simple and sensitive sensing strategy for thrombin in buffer solution and human serum samples. The key features of this assay lie in target binding-induced DNA hybridization and the formation of gold nanoparticle (AuNP) aggregation. In the presence of thrombin, the binding of the two DNA strands to the same target triggers the hybridization between the complementary sequences of the two DNA strands. The hybridization is able to pull a few AuNPs together to form oligomers. A linear dependence between the absorbance and target thrombin concentration is obtained under optimal conditions in the range from 10 nM–5 μM with a detection limit (LOD) of 7.5 nM estimated at the 3S blank level. Moreover, this method is successfully applied to complex serum samples without complicated sample pretreatment and sophisticated instruments, a dynamic range (10 nM–5 μM) and a detection limit as low as 9.1 nM is achieved. [ABSTRACT FROM AUTHOR]

A facile colorimetric sensor array for detection of proteins was demonstrated using DNA as nonspecific receptors. We found that different proteins could trigger the DNA-protected gold nanoparticles (AuNPs) to show different aggregation behaviors in the presence of salt with high concentrations along with various color changes. As a proof-of-concept application, a multi-protein discrimination array was fabricated with two ssDNA strands (15A and 30A bases) as a receptor array. The combinatorial colorimetric response of this sensor array can be analyzed by linear discriminant analysis (LDA) and hierarchical cluster analysis (HCA). 12 proteins have been well distinguished with the naked eye at the 50 nM level. The proteins in human serum have also been discriminated. Furthermore, the accuracies of discrimination of the similar mixtures of proteins and unknown samples were all 100%. [ABSTRACT FROM AUTHOR]

Mercury ion (Hg 2+ ) has been recognized as one of the most hazardous metal pollutants that can cause deleterious effects on human health and the environment even at low concentrations. Therefore, monitoring of Hg 2+ levels in water is very important in terms of waste management, environmental analysis, toxicology, water safety, and water quality. Here, we provide a highly sensitive and selective colorimetric assay to detect Hg 2+ in aqueous solution using iodide responsive core-shell Cu@Au nanoparticles (Cu@Au NPs). Iodide can induce an appreciable color change of the Cu@Au NP solution from gray to red by transforming the interconnected, irregularly shaped nanoparticles to the single, separated, and nearly spherical ones. In the presence of Hg 2+ , the formation of Cu@Au-Hg nanoalloys can protect the Cu@Au NPs from absorbing more iodide in the solution, the solution color turns pale red. When Hg absorbed on the surface of Cu@Au NPs reaches saturation, Hg prevents iodide from reacting with Cu@Au NPs, thus the solution color remains gray. This method exhibits a wide response range from 0.1 nM to 70 nM, and the detection limit (LOD) of 0.037 nM is obtained. Besides, our Cu@Au NPs-based colorimetric sensor exhibits excellent selectivity for Hg 2+ . [ABSTRACT FROM AUTHOR]

We have developed aptamer-functionalized gold nanoparticles (AuNPs) for the rapid, selective, and sensitive detection of Ag + in aqueous solution. Two DNA strands are designed to functionalized on the surface of AuNPs through Au S bond. In the presence of Ag + , aptamer1 hybridizes with the partially complementary aptamer2 with the aid of C Ag + C bindings, which could lead to different degrees of AuNP aggregation. This process is accompanied by a color change from red to bluish-purple as observed by naked eyes. This colorimetric sensor is shown to detect the presence of as low as a 0.236 nM concentration of Ag + . Importantly, it could be applied to complex samples without complicated sample pretreatment and sophisticated instruments. [ABSTRACT FROM AUTHOR]

An ultrasensitive colorimetric assay for Hg 2+ detection was presented based on synergistic catalytic effect of gold nanoparticles (AuNPs) and Hg. The AuNPs could catalyze the reduction of Hg 2+ by surface-capping citrate. The effect of Hg 2+ uptake implied amalgam formation and increased the surface area of AuNPs, leading to remarkable catalytic amplification of the peroxidase-like activity of AuNPs. On the basis of this effect, 3,3′,5,5′-tetramethylbenzidine (TMB) as colorimetric probes could be catalyzed into a black blue product in the presence of H 2 O 2 . The detection limit of the colorimetric assay for Hg 2+ is 0.1 nM. Furthermore, the detection limit of Hg 2+ in river water samples and tap water samples are 0.66 nM, and 0.43 nM, respectively. The results are far below the World Health Organization (WHO) permissible limit (30 nM). The assay is highly sensitive, specific, and able to detect Hg 2+ rapidly without requiring sophisticated equipment. In addition, our strategy could also be used as a promising and convenient approach for detection of the target in environmental protection. [ABSTRACT FROM AUTHOR]

Flavonoids are closely related to human health. The identification and determination of flavonoids is an important and difficult issue. In view of this, this work puts forward a colorimetric sensing array for flavonoid discrimination based on gold nanoparticle (AuNP) aggregation. Two substances (acetylthiocholine iodide (Atc) and S-propionylthiocholine iodide (Ptc)) are hydrolyzed to choline (Chi) under the action of acetylcholinesterase (AcChE). Choline can agglomerate AuNPs by forming Au S bond with its own sulfhydryl functional group. The presence of flavonoids inhibits the AcChE activity, resulting in the formation of different amounts of Chi and different degrees of AuNP aggregation. Controlled by Atc and Ptc as array's receptors, the five flavonoids, including proanthocyanidin (Pro), naringenin (Nar), quercetin (que), flavone (Fla), and curcumin (Cur), were differentiated through their unique colorimetric "fingerprint" pattern-based recognition with the aid of linear discriminant analysis (LDA). The assay is validated under phosphate buffered saline (PBS) buffer conditions as well as in human serum. In addition, identification for diverse concentrations of single flavonoid and different binary and ternary mixtures of flavonoids were also demonstrated. • The identification and determination of flavonoids is an important and difficult issue. • A colorimetric sensor array for flavonoid discrimination based on gold nanoparticle aggregation is developed. • The presence of flavonoids inhibits the AcChE activity, resulting in different degrees of AuNP aggregation. • Identification for diverse concentrations of single flavonoid and flavonoid mixtures were demonstrated. [ABSTRACT FROM AUTHOR]

• We propose a simple but sensitive colorimetric method for the detection of P 2 O 7 4−and PPase. • By counting the IOD of red dots in the dark-field images, P2O74− concentration can be determined. • The linear range of P2O74−concentrations (0.1 nM - 10 mM) and a LOD (0.0324 nM) were obtained. We propose a sensitive colorimetric method for the detection of P 2 O 7 4− and pyrophosphatase (PPase). Without Na 4 P 2 O 7 and PPase, under the catalysis of gold nanoparticles (AuNPs), Cu(II) can be reduced to Cu by nicotinamide adenine dinucleotide (NADH), leading to the Cu-coating onto the surfaces of AuNPs, accompanied by the color change from green to red under the dark-field microscope observation. While in the presence of Na 4 P 2 O 7 , the formation of Au@Cu NPs is suppressed due to the strong complexation reaction between Cu(II) and Na 4 P 2 O 7. Thus, the NPs exhibit uniform green color in the dark-field image. By quantitatively counting the integrated optical density (IOD) of red dots in the dark-field images, P 2 O 7 4− concentration can be accurately determined. A linear logarithmic dependence on P 2 O 7 4− concentrations from 0.1 nM to 10 mM with a limit of detection (LOD) of 0.0324 nM was obtained. The addition of PPase to the Na 4 P 2 O 7 -triggered dispersed AuNPs restores the formation of Au@Cu NPs owing to the hydrolysis of Na 4 P 2 O 7 into sodium phosphate under the catalysis of PPase, resulting in reliable quantification of color change of NPs from red to green in the dark-field images. The LOD is as low as 0.0007 U/mL with a linear dynamic range of 0.005–10 U/mL. [ABSTRACT FROM AUTHOR]