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Thiocyanate (SCN − ) is a small anion byproduct of cyanide metabolism. Several methods have been reported to measure SCN − above the micromolar level. However, SCN − is derived from many sources such as cigarettes, waste water, food and even car exhaust and its effect is cumulative, which makes it necessary to develop methods for the detection of trace SCN − . In this paper, a simple and ultrasensitive turn-on fluorescence assay of trace SCN − is established based on the fluorescence resonance energy transfer (FRET) between gold nanoparticles (AuNPs) and fluorescein. The detection limit is 0.09 nM, to the best of our knowledge, which has been the lowest detection LOD ever without the aid of costly instrumentation. The fluorescence of fluorescein is significantly quenched when it is attached to the surface of AuNPs. Upon the addition of SCN − , the fluorescence is turned on due to the competition action between SCN − and fluorescein towards the surface of AuNPs. Under an optimum pH, AuNPs size and concentration, incubation time, the fluorescence enhancement efficiency [( I F − I 0 )/ I 0 ] displays a linear relationship with the concentration of SCN − in the range of 1.0 nM to 40.0 nM. The fluorescein–AuNP sensor shows absolutely high selectivity toward SCN − than other 16 anions. The common metal ions, amino acids and sugars have no obvious interference effects. The accuracy and precision were evaluated based on the recovery experiments. The cost effective sensing system is successfully applied for the determination of SCN − in milk products and saliva samples. [ABSTRACT FROM AUTHOR]

A colorimetric assay for acid phosphatase (ACP) was constructed that is based on in situ polymerization of aniline catalyzed by gold nanoparticles (AuNPs). Aniline can be polymerized by ammonium persulfate (APS) in acidic condition and form gold-polyaniline core-shell nanoparticles (Au@PANI NPs) in the presence of AuNPs with the assistance of sodium dodecyl sulfate (SDS). AuNPs were also found to accelerate the polymerization process of aniline and thus shorten the reaction time. Upon the introduction of ascorbic acid (AA), the oxidant APS was consumed via the redox reaction. That led to the suppression of the formation of PANI. Consequently, ACP activity can be supervised on the basis of hydrolysis of 2-phospho-L-ascorbic acid trisodium salt (AAP) catalyzed by ACP to release AA. With the increase of ACP activity, the intensity ratio of the absorbance at λ705 nm (A705) and the absorbance at λ530 nm (A530) gradually decreased and the color gradually changed from dark-green to light-green to blue-gray to purple and eventually to pink. This method for ACP determination worked in the range 0.40 to 2.00 U·L−1. The detection limit is 0.043 U·L−1. The assay was applied to determine ACP in human serum. The recovery ranged from 81.0 to 104.6%. Relative standard deviation was less than 5%. This suits the request for biological sample analysis. [ABSTRACT FROM AUTHOR]

A simple but efficient colorimetric assay was developed for the detection and quantification of acid phosphatase (ACP) using a smartphone. This strategy is based on target-controlled iodine-mediated etching of gold nanorods (AuNRs). Due to effective hydrolysis of the substrate pyrophosphate (PPi) by ACP, chelated Cu2+ with PPi was released, which promoted the redox reaction with an iodide ion (I−), leading to the formation of I3−. As the etching agent of AuNRs, I3− caused a blueshift of the localized surface plasmon resonance peak and, more importantly, an observable color change. The vivid colors were recorded with a smartphone camera and directly analyzed using an image-processing app. On the basis of the direct correlation between ACP concentration and the etching degree of AuNRs as well as color change, this smartphone nanocolorimetry technique showed a good linear response toward ACP over the range of 0–15.0 U/L, with a detection limit of 0.97 U/L. Using the standard addition method, the practical applicability of the proposed smartphone-based assay was successfully demonstrated by determining ACP in human serum samples, with results consistent with those obtained by UV-Vis spectrophotometry. [ABSTRACT FROM AUTHOR]

Glutathione (GSH) plays a key role in biological systems and serves many cellular functions. Since biothiols all incorporate thiol, carboxylic and amino groups, discriminative detection of GSH over cysteine (Cys) and homocysteine (Hcy) is still challenging. Both Cys and Hcy can induce aggregation of AuNPs via the formation of zwitterionic networks, which is different from GSH. GSH induced aggregation of AuNPs by the hydrogen bonding between the carboxylic groups when pH <4.5, so pH plays a crucial part in our present work. By regulating the pH to 5.8, our proposed sensing system exhibits high selectivity and sensitivity towards GSH even in the presence of other biothiols, realizing the discriminative detection of GSH over Cys/Hcy based on anti-aggregation of AuNPs. In our work, 2-mercapto-1-methylimidazole (MMI) and GSH took the role of aggregation and anti-aggregation agent, respectively. A linear relationship between the intensity ratio A 650 nm /A 520 nm and the GSH concentration is obtained (R 2 = 0.998) with a detection limit of 12.0 nM. [ABSTRACT FROM AUTHOR]

Hypoxia refers to the lack of oxygen supply to cells or tissues. The overexpression of nitroreductase has been shown to be closely related to the degree of hypoxia, which leads to the level of nitroreductase (NTR) being used as an indicator of hypoxia. We reported a facile visual detection of NTR based on the aggregation of gold and silver alloy nanoparticles. Compared with gold nanoparticles (AuNPs), the aggregation behavior of Au 80 Ag 20 NPs caused a more prominent color change. Copper ions (Cu2+) can be rapidly reduced by nicotinamide adenine dinucleotide (NADH) under the catalysis of Au 80 Ag 20 NPs. But NADH is consumed as an electron donor during the catalytic reduction reaction of p-nitrophenol (pNP) by NTR. A decrease of NADH amount results in the aggregation of Au 80 Ag 20 NPs by the excess Cu2+ and different aggregation degrees of Au 80 Ag 20 NPs lead to observable color change. A linear correlation of A 600 /A 505 = 0.0285 [NTR]+0.361 (R2 = 0.980) was obtained with a limit of detection (LOD) of 0.23 μg/mL for UV–vis spectrophotometer. For visual detection, the values of R/B against the concentration of NTR obtains a calibration curve of R/B = −0.031 [NTR]+ 1.54 (R2 = 0.985) with a LOD of 0.76 μg/mL, which is of the same order of magnitude as the UV–vis spectrophotometer analysis. As a comparison, Au 80 Ag 20 NPs was replaced by several different composition nanoparticles (Au NPs, Au 70 Ag 30 NPs, Au 50 Ag 50 NPs) to be a chromogenic substrate, and the results suggest the Au 80 Ag 20 NPs is the most sensitive substrate in our assay. Selectivity tests showed that the detection system did not respond to other common substances, and the reaction mechanism was verified by inhibitor research. Finally, the assay was used on the human serum samples with spiking NTR, and the recovery rates of this assay with UV–vis spectrophotometer were basically consistent with RGB analysis. In the absence of NTR, NADH can reduce Cu2+ to Cu (0) under the catalysis of the Au 80 Ag 20 NPs. When NTR is introduced, NADH is consumed in the catalytic reaction, and Cu2+ that cannot be reduced will cause the aggregation of Au 80 Ag 20 NPs due to the strong affinity between Cu2+ and citrate. [Display omitted] • Our assay is a rapid and simple colorimetric detection for nitroreductase. • Visual detection of nitroreductase can be accompanied due to the Au x Ag y NPs property. • The sensitivity of the assay by the naked eye is comparable to that of the apparatus. [ABSTRACT FROM AUTHOR]

Gold nanoparticles (AuNPs) catalyze the mild reaction between the weak reducing agent kojic acid (KA) and silver ions (Ag+) to form Au@Ag bimetallic NPs by the combination of the intrinsic catalysis with plasmonic properties This is proposed as a novel optical assay to determine the tyrosinase (TYRase) concentration. The nanoparticles have been characterized by UV-vis spectroscopy, transmission electron microscope (TEM) images, and X-ray photoelectron spectroscopy (XPS). The sensing mechanism is based on the fact that KA binds to TYRase by chelating with dicopper active site of TYRase and the introduction of TYRase restrains the Au@Ag bimetallic NP formation by the precedent binding with KA. A clear color variation from yellow to pink and UV-vis spectral changes are observed at the optimal wavelength of 410 nm. The assay works in the range 0.13~0.73 U mL−1 with a detection limit (LOD) of 0.019 U mL−1. The impact from matrix interfering substances including glucose, uric acid, common oxidases, and amino acids is negligible. The applicability is demonstrated by quantitative determination of TYRase in human serum samples with 74 to 89% recovery and RSD less than 4.0%, which accords with the level for bio-sample analysis. [ABSTRACT FROM AUTHOR]

An optical nanoprobe consisting of gold nanoclusters (AuNCs) and gold nanoparticles (AuNPs) is described for ultrasensitive detection of heparin (Hep). Polyethyleneimine (PEI) induces the aggregation of AuNPs which results in a color change from wine red (peak at 520 nm) to blue (peak at 610 nm). In parallel, the fluorescence of AuNCs (with excitation/emission maxima at 370/610 nm) is weakened. However, in the presence of Hep (which is strongly negatively charged), it will electrostatically bind to positively charged PEI and then will prevent aggregation. Hence, the color changes from blue (aggregated) to red (non-aggregated). In parallel, fluorescence remains unchanged. Hep can be quantified by using the nanoprobe in the range of 4–220 ng·mL−1, with the detection limits as low as 1.6 (colorimetry) and 3.4 ng·mL−1 (fluorometry). The assay was applied to the detection of Hep in (spiked) human serum with satisfactory results. [ABSTRACT FROM AUTHOR]

A label-free, rapid response colorimetric aptasensor for sensitive detection of chloramphenicol (CAP) was proposed, which was based on the strategy of ssDNA-modified gold nanoparticle (AuNP) aggregation assisted by lanthanum (La3+) ions. The AuNPs generated a color change that could be monitored in the red, green, and blue and analyzed by the smartphone imaging app. La3+, as a trigger agent, strongly combined with the phosphate groups of the surface of ssDNA-AuNPs probe, which helps create AuNP aggregation and the color change of AuNPs from red to blue. On the contrary, when mixing with CAP, the aptamer (Apt) bound to CAP to form a rigid structure of the Apt-CAP complex, and La3+ attached to the phosphate groups of the complex, which prevented the aptamer from binding to the surface of the AuNPs. As a result, the color of the AuNPs changed to violet-red. Finally, UV-vis absorption spectroscopy and the smartphone imaging app were employed to determine CAP with a lower detection limit of 7.65 nM and 5.88 nM, respectively. The proposed strategy featuring high selectivity and strong anti-interference ability for detection of CAP in practical samples was achieved. It is worth mentioning that the simple and portable colorimetric aptasensor will be used for facilitating on-site detection of food samples. [ABSTRACT FROM AUTHOR]

Detecting cancer biomarker levels in body fluids is essential for medical diagnosis. Enzyme-linked immunosorbent assay (ELISA) has been broadly used to detect cancer biomarkers. However, colorimetric ELISA based solely on nanoparticles (NPs) are susceptible to environmental influences, which often results in the detection inaccuracy, being limited in clinical applications. In this regard, the dual-mode approach would add signal diversity to the detection, making the results more reliable. We present colorimetric and photothermal immunosensor that enables direct reading of the color and temperature of the solution. A core-satellite nanoprobe constructed by polydopamine (PDA) as the core and gold seeds as satellites is rationally designed as the signal reporter. When ascorbic acid is present in the solution, PDA can cooperate with ascorbic acid to reduce chloroauric acid and mediate the growth of gold seeds on the PDA surface, inducing a redshift of the localized surface plasmon resonance peak of the nanosensor and the change in photothermal conversion efficiency. The method is further combined with the sandwiched immunoassay to construct an alkaline phosphatase based colorimetric and photothermal ELISA for the highly sensitive and accurate evaluation and detection of prostate-specific antigen (PSA). The linear range was from 0.05 to 100 ng mL−1 with a detection limit of 6.71 pg mL−1 for the colorimetric detection, while the linear range was from 0.5 to 90 ng mL−1 with a detection limit of 0.13 ng mL−1 in the photothermal analysis. The accurate detection of PSA levels in serum samples was well demonstrated with the dual-mode approach. The presented immunoassay allows straightforward, sensitive, and selective readout by color and temperature without advanced instrumentation. Particularly, the LOD was much lower than the threshold in clinical trials for PSA. Therefore, this method has a great prospect in the early diagnosis of cancer biomarkers based on a dual-mode multifunctional platform. [Display omitted] • We developed a dual-mode immunosensor based on gold nanoparticle growth. • The nanoprobe showed color and temperature changes caused by the cancer biomarker. • The assay had high detection sensitivity and accuracy. • The strategy was used to assess prostate-specific antigen levels in serum samples. [ABSTRACT FROM AUTHOR]

• A colorimetric aptasensor for the detection of multiple antibiotics is presented. • The method allows for multiplex antibiotics detection by the naked eye. • This colorimetric aptasensor displays great tolerance to high salt concentrations. • The method would promote the practical application via RGB analysis of smartphone. We devise a novel colorimetric aptasensor for multiplex antibiotics based on an ss-DNA fragment coordinately controlling gold nanoparticles (AuNPs) aggregation. The multifunctional aptamer (Apt) was elaborately designed to be adsorbed on AuNPs surfaces acting as a binding element for antibiotics and a molecular switch. Chloramphenicol (CAP) and tetracycline (TET) were selected as the model antibiotics. When one kind of antibiotics was added, the specifically recognized fragment of Apt can bind to it and dissociated, and the non-specific one coordinately controls AuNPs aggregation under high-salt conditions. Hence, different color changes of AuNPs solution can be used as the signal readout. The aptasensor exhibited remarkable selectivity and sensitivity for separate detection of TET and CAP, and the detection limits are estimated to be 32.9 and 7.0 nM, respectively. The analysis with the absorption spectroscopy and the smartphone are applied to detect antibiotics in real samples with consistent results and desirable recoveries. [ABSTRACT FROM AUTHOR]

Exosomes, as potential cancer diagnostic markers have received close attention in recent years. However, there is still a lack of simple and convenient methods to detect and quantitate exosomes. Herein, we used a simple paper-supported aptasensor based on luminescence resonance energy transfer (LRET) from upconversion nanoparticles (UCNPs) to gold nanorods (Au NRs) for the accessible determination of exosomes. When exosomes are present, the two sections of the aptamer can combine with the CD63 protein on the surface of exosomes and form a conjugation to close the distance between UCNPs and Au NRs, which initiates the LRET and promotes luminescence quenching. These variations can be monitored by the homemade image system, and the green channel intensities of obtained colored images were extracted with photoshop software to quantify the luminescence. As a result, the quenching of the luminescence of the UCNPs is linearly correlated to the concentration of the exosomes (in the range of 1.0 × 10 4 ~ 1.0 × 10 8 particles/μL), enabling the detection and quantification of the exosomes. Such approach can reach a low detection limit of exosomes (1.1 × 10 3 particles/μL) and effectively reduce the background signal by using UCNPs as a luminescent material. This study provides an efficient and practical approach to the detection of exosomes, which should lead to point-of-care testing in clinical applications. [ABSTRACT FROM AUTHOR]

A rapid method for the determination of methionine (Met) based on the anti-aggregation of gold nanoparticles (AuNPs) in serum and urine is presented. Melamine (Mel) can induce the aggregation of AuNPs through the ligand exchange between amine groups on melamine and negatively charged citrate ions and then induce color change of the AuNPs solution from wine red to blue. However, the AuNPs is more prone to combining with Met through both Au S and Au N bonds in the presence of Mel, which hinders the aggregation of AuNPs induced by Mel. A good linear correlation (R 2 = 0.996) between absorbance ratio and the Met concentration is obtained over the range from 0 μM to 1.0 μM. The detection limit was estimated as low as 24.5 nM and the recovery of Met added to serum and urine samples was 95.0-105% and 95.0-104%, respectively. The result represents that the method makes this sensor promising for the applications in the development of detecting Met. [ABSTRACT FROM AUTHOR]

A facile, highly sensitive and selective colorimetric strategy for thiocyanate (SCN − ) detection is proposed based on anti-aggregation of gold nanoparticles (AuNPs) which was induced by surfactant cetyltrimethyl ammonium bromide (CTAB). A certain concentration of aggregation agent CTAB is responsible for a visible color change of AuNPs from wine red to blue. The aggregations of AuNPs decrease with increasing concentrations of SCN − , which can be monitored by an ultraviolet–visible spectrophotometer or the naked eyes. The limit of the detection (LOD) is 0.1 μM by the naked eyes and 6.5 nM by UV–vis spectroscopy with the linear range from 0.1 to 1.5 μM. The applicability of our detection system is also verified by analysis of SCN − in milk samples and satisfactory results were obtained. [ABSTRACT FROM AUTHOR]

A new method for real-time detection of carcinoembryonic antigen (CEA) in human serum with high sensitivity and selectivity using surface plasmon resonance (SPR) biosensor was developed. Two kinds of antibodies were used to recognize CEA at different epitopes with high affinity and specificity. Gold nanoparticles (GNPs) modified with streptavidin (SA) were used to further enhance signal specifically via biotin–streptavidin interaction. The binding capacity of the streptavidin-modified gold nanoparticles (SA–GNPs) for ligand biotin was quantified by titration with biotin (5-fluorescein) conjugate to be 10.54 biotin binding sites per 100 nm 2 . The developed GNPs enhanced sandwich SPR biosensor successfully fulfilled the sensitive detection of CEA in the range of 1–60 ng/mL with a detection limit of 1.0 ng/mL. Compared to the direct assay format, sandwich format without GNPs and SA–GNPs enhanced sandwich format led to 4.2-fold and 13.8-fold in the sensitivity, respectively. This sensor also showed good selectivity for CEA in the interference study. The results demonstrated that the proposed method could provide a high sensitivity and selectivity in the detection of CEA and offer a promising alternative for cancer biomarker than traditional clinical examinations. [ABSTRACT FROM AUTHOR]

The superior supramolecular recognition ability of macrocyclic compounds will enhance the sensitivity and selectivity of electrochemical detection, which has a great application potential in electrochemical sensing. Herein, we developed a novel electrochemical aptasensor based on the specific host-guest interactions between cucurbit [7]uril and ferrocene (Fc) for capture, determination and release of exosomes. Macrocyclic compounds, cucurbit [7]uril is modified on the surface of the gold nanoparticles composed electrode by self-assembling. CD63 aptamer linked ferrocene is introduced into this platform to capture exosomes specifically by CD63 protein on the exosomes. The dual specificity of macrocyclic compounds and aptamers enables highly selective and sensitive electrochemical detection of exosomes. The limit of detection (LOD) was 482 particles μL-1. In addition, the captured exosomes could be released on demand in a very mild manner through aminoferrocene (NH 2 -Fc) because of its higher affinity to cucurbit [7]uril. The proposed electrochemical aptasensor showed good performance in detecting exosomes even in plasma samples, thus demonstrating its great potential in early clinical diagnosis. Simultaneously, exosomes could be released undamaged by this protocol, exhibiting good applicability in comprehensive studies of exosomes. Moreover, this strategy can be applied to other target biomolecules by changing the recognition pairs. Specific host-guest interactions between cucurbit [7]uril and ferrocene (Fc) based electrochemical aptasensor was developed to capture and release exosomes. [Display omitted] • A novel host-guest interactions based electrochemical aptasensor was developed for exosomes capture, determination and release. [ABSTRACT FROM AUTHOR]

• A ratiometric fluorescent nanoprobe for phosphate was developed. • The response to phosphate is based on chelation-enhanced fluorescence. • The self-calibrating capacity can well alleviate environmental interference. • This probe showed excellent selectivity and sensitivity toward phosphate. • This probe was applied to the quantitation of phosphate in human serum. We report a new fluorescent nanoprobe simply constructed by the surface modification of positively charged polyethylenimine (PEI)-coated gold nanoclusters (AuNCs) with 8-hydroxyquinoline-5-sulfonate (8Q5S) through electrostatic interaction. This nanoprobe showed the intrinsic emission of AuNCs at 650 nm and a weak emission at 490 nm from 8Q5S. Based on chelation-enhanced fluorescence of 8Q5S induced by its specific and high affinity for phosphate upon binding Mg2+ and the stable fluorescence of AuNCs as the reference, this nanoprobe was demonstrated to exhibit highly sensitive, selective, and reliable detection of phosphate in the linear range from 0 to 50 μM with a low detection limit of 0.14 μM. Furthermore, the presented ratiometric fluorescent sensor was successfully employed for accurate quantitation of phosphate in complex human serum samples with satisfactory results. [ABSTRACT FROM AUTHOR]

A colorimetric assay is described for determination of cytosine-rich ssDNA at physiological pH values. The working principle is based on (a) Ag(I) ion-induced formation of an i-motif structure, and (b) glucose oxidase-controlled growth of gold nanoparticles (AuNPs). The combination between Ag+ and cytosine-rich DNA can modulate the generation of H2O2 resulting from enzyme catalyzed glucose oxidation. Depending on the amount of H2O2 formed, the solution containing the AuNPs will turn red in the presence of cytosine-rich ssDNA but blue in the absence of such DNA if Ag+ is added before the formation of the red AuNPs. Upon addition of C-DNA at different concentrations, the peak shift (Δλ) of the AuNP solution relative to the SPR peak position (560 nm) in the absence of C-DNA is taken as the signal readout. The method shows a good linear response toward C-DNA over the range 10–200 nM with a detection limit of 2.7 nM. It may also be performed visually. The photometric assay is highly sensitive, specific, and rapid. The method is particularly attractive in terms of applications such as in human serum analysis, a colorimetric logic gate, and the calculation of binding constants for the interaction between Ag+ and glucose oxidase (GOx), and between Ag+ and cytosine-rich ssDNAs. [ABSTRACT FROM AUTHOR]