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We herein present a simple, low-cost, and ultrasensitive colorimetric sensing strategy for the detection of mercury ions (Hg2+) that takes advantage of the natural pore structure in rose petals to encapsulate gold nanoparticles (AuNPs). AuNPs easily aggregate in high salt concentrations, which restrict their application, for example, as a catalyst in real sample analysis. Rose petals offer a three-dimensional (3D) space for more efficient dispersability of AuNPs in their extensive network of channels, which will increase the specific surface area, thereby enhancing the catalytic capability of AuNPs. The presence of Hg2+ triggers the formation of gold amalgam, which results in remarkable enhancement of the catalytic ability of citrate-capped AuNPs. On the basis of this effect, a colorimetric signal amplification is achieved via the use of gold amalgam to catalyze the reaction of methylene blue (MB) and sodium borohydride. A limit of detection (LOD) as low as 10 fM can be obtained by naked eye observation. Our sensor has promising potential for detection and quantitation of Hg2+ in environmental samples. [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]

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 study, a simple and highly sensitive colorimetric assay of H 2 O 2 using triangular Ag nanoplates has been carried out. In the presence of H 2 O 2 , the Ag nanoplates are etched from triangle to round, which leads to the vanishment of the surface plasmon resonance (SPR) absorption at 400 nm and significant decrease the absorbance of the band at 670 nm. The color change of the reaction system from the initial blue to mauve can be observed. This proposed method can determine H 2 O 2 with a dynamic range of 50 nM to 1 μM and a low detection limit of 1.36 nM. The high sensitivity of the triangular nanoplates can be ascribed to highly reactive edges/tips, and strongly tip sharpness and aspect ratio dependent SPR absorption of the triangular nanostructure. This sensing system is further extended to the detection of glucose by H 2 O 2 generated from the enzymatic oxidation of glucose, and thus holds great promises in clinical applications. [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]

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]

Abstract We develop a glucose oxidase (GOx)-mediated strategy for detecting glucose based on oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB), which is generated from Cu2+-catalyzed 3,3′,5,5′-tetramethylbenzidine (TMB)-H 2 O 2 reaction, as colorimetric readout. The sensing system involves two processes: generation of H 2 O 2 from GOx-catalyzed oxidation of glucose, and H 2 O 2 -induced the oxidization of TMB via the catalysis of Cu2+. The H 2 O 2 formed by GOx-catalyzed oxidation of glucose oxidizes colorless TMB to blue oxTMB, thus enhancing the absorbance intensity at 670 nm. Therefore, we draw a conclusion that the enhancement in colorimetric signal relies directly on H 2 O 2 concentration, which, in turn, depends on glucose concentration. This color change can be used not only for visual detection of glucose by naked eyes but for reliable glucose quantification in the range from 1 to 100 nM with a detection limit of 0.21 nM. The method possesses the following advantages: simple design, low experimental cost, and no any additional experimental equipment for heating, illuminating, or bubbling. Graphical Abstract Unlabelled Image Highlights • A GOx-mediated strategy for detecting glucose based on oxTMB, which is generated from Cu2+-catalyzed TMB-H 2 O 2 reaction. • The enhancement in colorimetric signal relies directly on H 2 O 2 concentration. • The method exhibits an ultrasensitive detection of glucose with the detection limit down to 0.21 nM. • The method is universally applicable for detection of other H 2 O 2 -mediated substrates and enzymes. [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]

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]

In the present work, we report a novel colorimetric sensor array for rapid identification of heavy metal ions. The sensing mechanism is based on the competition between thiols and urease for binding with the metal ions. Due to the different metal ion-binding abilities between the thiols and urea, different percentages of urease are free of metal ions and become catalytically active in the presence of varied metal ions. The metal ion-free urease catalyzes the decomposition of urea releasing ammonia and changing the pH of the analyte solution. Bromothymol blue, the pH indicator, changes its color in response to the metal-caused pH change. Three different thiols (L-glutathione reduced, L-cysteine, and 2-mercaptoethanol) were used in our sensor array, leading to a unique colormetric repsonse pattern for each metal. Linear discriminant analysis (LDA) was employed to analyze the patterns and generate a clustering map for identifying 11 species of metal ions (Ni2+, Mn2+, Zn2+, Ag+, Cd2+, Fe3+, Hg2+, Cu2+, Sn4+, Co2+, and Pb2+) at 10 nM level in real samples. The method realizes the simple, fast (within 30 s), sensitive, and visual discrimination of metal ions, showing the potential applications in environmental monitoring. [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]

In this work, we report an ultrasensitive colorimetric sensing strategy for Cu 2+ based on the Cu 2+ catalyzed TMB-H 2 O 2 reaction, and the signal amplification induced by aptamer-K + -Cu 2+ complex. It is demonstrated that Cu 2+ exhibits an intrinsic peroxidase-like catalytic activity. Thus, we utilize the catalytic activity of Cu 2+ to TMB-H 2 O 2 reaction for direct colorimetric visualization of Cu 2+ for the first time. The limit of detection (LOD) is 2.6 μM. To further improve the sensitivity of Cu 2+ detection, we utilized G-quadruplex-Cu 2+ metalloenzyme complex formed by K + , G-rich DNA, and Cu 2+ to amplify colorimetric signal of TMB-H 2 O 2 reaction. The LOD is as low as 0.076 μM, which is 34-fold lower than that of only Cu 2+ -catalyzed TMB-H 2 O 2 system (2.6 μM). The LOD shows excellent potential for monitoring ultralow levels of Cu 2+ in water samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • We developed a sensor array based on Co-N-C single-atom nanozymes for colorimetric discrimination of antioxidants. • Under two pH conditions, Co-N-C nanozymes produce different catalytic effects on the reaction between H 2 O 2 and TMB. • Linear discriminant analysis (LDA) was used to successfully analyze the 7 antioxidants. • The sensing strategy had excellent specificity and good application in human serum samples. Here we develop a simple and effective nose/tongue sensor array based on Co-N-C single-atom nanozymes-3,3′,5,5′-tetramethylbenzidine (TMB)-H 2 O 2 for colorimetric discrimination of antioxidants, which makes use of the color reaction of TMB oxidation by H 2 O 2 in two different pH (3.8 and 4.6) environments under the catalysis of Co-N-C nanoenzyme with peroxidase-like activity. Different antioxidants have varying reducing ability to the oxidation products of TMB (oxTMB), thus resulting in differential absorbance and color changes. Linear discriminant analysis (LDA) results indicate that the sensor array successfully identified 7 antioxidants, i.e., glutathione (GSH), ascorbic acid (AA), cysteine (Cys), tannin (TA), Catechin (C), dopamine (DA), and uric acid (UA) in both buffer and even serum samples. Additionally, the performance of the sensor array was validated with antioxidant mixtures, individual antioxidants with different concentrations, and target antioxidants and interfering substances. In general, the versatile sensor array based on Co-N-C single-atom nanozymes provides an excellent strategy for identifying a variety of antioxidants, which exhibits a broad application prospect in medical diagnosis. [ABSTRACT FROM AUTHOR]