Bifunctional Tb(III)-modified Ce-MOF nanoprobe for colorimetric and fluorescence sensing of α-glucosidase activity.

α-Glucosidase, which directly involves in the metabolism of starch and glycogen and causes an increase in blood sugar level, is the major target enzyme for the precaution and therapy of type II diabetes. Based on the previous work, we adopted a post-synthetic modification method to encapsulate Tb3+ into Ce-MOF nanozyme which owned mixed valence states. Tb@Ce-MOF displayed induced luminescence characteristic and exceptional oxidase-like activity that could oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue ox-TMB. α-Glucosidase can hydrolyze the substrate l-ascorbic acid-2-O-α-d-glucopyranosyl (AAG) to generate ascorbic acid (AA), which could increase the Ce3+/Ce4+ redox valence mode in Tb@Ce-MOF, leading to the inhibition of the allochroic reaction of TMB and the decreased absorption of ox-TMB at 652 nm. The energy transfer (EnT) process from Ce3+ to Tb3+ will enhance due to the increased Ce3+/Ce4+ mode in Tb@Ce-MOF, which will result in an enhanced fluorescence signal of Tb@Ce-MOF at 550 nm. But the addition of inhibitor acarbose will inhibit the above process. We have constructed a dual-mode detection platform of α-glucosidase and its inhibitor via colorimetric and fluorometric method. The linear range of α-glucosidase were 0.01-0.5 U/mL (colorimetric mode) and 0.8-1.5 U/mL (fluorometric mode), respectively, with a detection limit as low as 0.0018 U/mL. Furthermore, our approach was also successfully employed to the analysis of α-glucosidase in serum samples.

UiO-66-NH2 initiated cascade reaction: Constructing a ratiometric fluorescence sensor for ultrasensitive detection of nerve agent simulant.

BACKGROUND: Highly toxic organophosphorus nerve agents often exist in the form of gas in the environment and can damage human neuroregulatory system by inhibiting the activity of acetylcholinesterase (AChE). However, fluorescent probes based on small organic molecules bring a secondary burden to environment, and their sensitivity and specificity for sarin simulant diethyl chlorophosphate (DCP) detection are unsatisfactory. Nanozyme cascade systems with signal amplification can be used for highly sensitive identification of analytes, but are rarely used in ratiometric analysis of DCP. Combination of enzyme cascades and ratiometric fluorescence ensures the accuracy and sensitivity of the output signal. RESULTS: We prepared a self-assembled nanohybrid (Ag-AuNCs@UiO-66-NH2) by metal-organic framework material and gold nanoclusters. On the one hand, UiO-66-NH2 with enzyme-like activity was used to hydrolyze DCP into diethyl phosphate (DEP) and chloridion (Cl-). Cl- hindered aggregation-induced enhanced emission (AIEE) of AuNCs by binding with Ag+ and decreased the fluorescence of AuNCs. On the other hand, ligand metal charge transfer effect (LMCT) of UiO-66-NH2 was blocked by DCP to enhance the fluorescence of UiO-66-NH2. Combining ratiometric analysis and nanozyme cascade reaction, an ultra-sensitive fluorescence sensor for detecting DCP was constructed, and ensured the accuracy of experimental results. In addition, Ag-AuNCs@UiO-66-NH2 was embedded into the agarose hydrogel substrate, the resulting agarose hydrogel film allowed quantitative assessment of DCP vapor and high sensitivity was demonstrated (detection limit as low as 1.02 ppb). SIGNIFICANCE: A strategy combining enzyme cascade with ratiometric fluorescence was proposed, which improved the accuracy and sensitivity of the analysis results. The soft-solid platform based on agarose hydrogel film was constructed to realize the quantitative monitoring of sarin simulant gas. The LOD value obtained in this work is much lower than the immediately life-threatening or health threatening concentration of sarin.

Zirconium-based metal-organic framework loaded agarose hydrogels for fluorescence turn-on detection of nerve agent simulant vapor.

Developing reliable sensors that accurately detect deadly chemical gases is critical to global security. Nerve agents are one of the most dangerous chemicals in the world and are often found in gaseous forms in the environment, which remain a challenge to detect because of their low levels. In this paper, a fluorescent probe based on a Zr-based metal-organic framework UiO-66-NH2 was proposed. The specific binding between the Zr-O site of UiO-66-NH2 and diethyl chlorophosphate (DCP) blocked the ligand-to-metal charge transfer (LMCT) process in UiO-66-NH2, thereby enabling the fluorescence turn-on detection of DCP. More importantly, a simple and portable hydrogel soft-solid platform (UiO-66-NH2@Aga) was constructed by incorporating UiO-66-NH2 into the formation process of agarose (Aga) hydrogel for fast and sensitive detection of gaseous DCP. When the hydrogel was exposed to a low concentration of DCP vapor, its fluorescence changed from colorless to bright blue, allowing visualization of the DCP gas for analysis. The UiO-66-NH2@Aga integrated solid-state platform showed an excellent response to DCP vapor in the detection range of 1.98 to 9.90 ppm and with a detection limit of 1.16 ppm. This work opened up a unique way to design a convenient, low cost and practical gas physical examination platform.

Colorimetric and photothermal immunosensor for sensitive detection of cancer biomarkers based on enzyme-mediated growth of gold nanostars on polydopamine.

BACKGROUND: 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. RESULTS: 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. SIGNIFICANCE: 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.

Distinct dual enzyme-like activities of Fe-N-C single-atom nanozymes enable discriminative detection of cellular glutathione.

Fe-N-C single-atom nanozymes readily achieved discriminative detection of glutathione (GSH) over other biothiols with similar structure due to the difference between POD-like and OXD-like activities regarding the kind of reactive oxygen species. This colorimetric sensor demonstrated the heterogeneity of GSH levels in different cells and accurately monitored cellular GSH fluctuation.

Integrated ratiometric luminescence sensing strategy based on encapsulation of guests in heterobinuclear lanthanide coordination polymer nanoparticles for glucose detection in human serum.

Lanthanide coordination polymers (LnCPs) can be used as a host platform to encapsulate functional guest molecules for the construction of integrated sensing platforms. In this work, two guest molecules, rhodamine B (RhB) and glucose oxidase (GOx), were successfully encapsulated in a heterobinuclear lanthanide coordination polymer synthesized by self-assembly of Ce3+, Tb3+ and adenosine monophosphate (AMP) to form RhB&GOx@AMP-Tb/Ce. Both guest molecules show good storage stability and minimal leakage. The higher catalytic activity and stability of RhB&GOx@AMP-Tb/Ce is obtained due to the confinement effect compared to free GOx. RhB&GOx@AMP-Tb/Ce exhibits superior luminescence based on the internal tandem energy transfer process of the nanoparticles (Ce3+→Tb3+→RhB). Glucose can be oxidized in the presence of GOx to form gluconic acid and H2O2. Subsequently, Ce3+ in the AMP-Tb/Ce host structure can be oxidized by H2O2 to Ce4+, thereby interrupt the internal energy transfer process and cause ratiometric luminescence response. Benefiting from the synergistic effect, the smart integrated luminescent glucose probe exhibits a wide linear range (0.4-80 µM) and a low detection limit (74.3 nM) with high sensitivity, selectivity and simplicity, enabling the quantitative detection of glucose in human serum. This work describes a good strategy to construct an integrated luminescence sensor based on lanthanide coordination polymers.

Exploring Zr-Based Metal-Organic Frameworks as Smart Electrochromic Sensors by Coordination-Driven Surface Engineering.

As emerging stimuli-responsive materials, electrochromic metal-organic frameworks (MOFs) are still not utilized in sensing applications due to difficulties in water stability, facile synthesis and functionalization, and efficient translation of specific recognition events. Here, we firstly find that a Zr-based MOF furnished with postsynthetically created viologen-like electron-deficient moiety was electrochromic active. With a coordination-driven surface engineering strategy where phosphate-containing biomolecules are tethered to Zr nodes of the MOF, fine tuning the interface electron transfer was readily achieved, thus benefitting for constructing smart electrochromic sensors through the combination of the sensitivity of electrochemistry with the visuality of colorimetry. Particularly, MOF-coated conductive films enabled label-free detection of phosphoproteins, and aptamer-functionalized ones responded specifically to the target. In two cases distinct color changes allow for visual quantification. This study represents the first example of MOF-based electrochromic sensors developed by an efficient strategy, indicating the generality to electrochromic counterparts for various sensing applications.

Dual-product synergistically enhanced ratiometric fluorescence assay for alkaline phosphatase activity using core-shell lanthanide-based nanoprobe.

We developed a new core-shell ratiometric fluorescent nanoprobe simply prepared by a seeded growth method. The strong luminescence arising from the shell formed by coordination self-assembly of an alkaline phosphatase (ALP) substrate, l-ascorbic acid 2-phosphate (AAP), and Tb3+, together with the weak fluorescence of the core-a metal-organic framework, UiO-66-NH2, constitutes an ideal dual-emission characteristic. Since AAP can be specifically cleaved by ALP, the well-formulated core-shell nanostructure was destroyed upon exposure to ALP. In this case, the luminescence of Tb3+ was quenched due to the inefficient antenna effect, while the fluorescence of UiO-66-NH2 was strengthened by the synergistical enhancement of dual hydrolysates to inhibit the ligand-to-metal charge transfer (LMCT) process. Using the dual signal response, this nanoprobe was employed for ratiometric fluorescence detection of ALP activity in the range of 0.05-0.6 U mL-1 with a detection limit of 0.018 U mL-1, accompanied by a discernible fluorescence color evolution from turquoise to blue. In virtue of good properties in accuracy, sensitivity, selectivity and simplicity, this assay enabled quantitative detection of ALP activity in human serum and efficient screening of ALP inhibitors. More Importantly, a smartphone-assisted paper-based sensing platform was designed for on-site visual analysis of ALP activity in human serum, which may be very promising as a facile, portable, and robust format to facilitate relevant biological and clinical applications.

A Reversible Fluorescent Probe Based on a Redox-Switchable Excited-State Intramolecular Proton-Transfer Active Metal-Organic Framework for Detection and Imaging of Highly Reactive Oxygen Species in Live Cells.

Detection and imaging of highly reactive oxygen species (hROS) in biological systems using fluorescent probes are critical for the study of physiological and pathological processes induced by hROS. Herein, we report a redox-active luminescent metal-organic framework (MOF), which incorporates a hydroquinone moiety that can undergo a reversible transformation from the hydroquinone to the quinone by hROS like •OH and ClO-. Moreover, the intrinsic fluorescence originating from the excited-state intramolecular proton transfer (ESIPT) property of the organic linker can be finely regulated during this redox-switchable process. A reversible fluorescent probe for hROS is thus developed. The presented probe shows a sensitive, selective, and reversible response to hROS due to the integration of excellent structural characteristics and unique spectral properties of the MOF. The detection limits of •OH and ClO- are 0.22 and 0.18 µM, respectively. Furthermore, with good photostability and super biocompatibility, this simple yet efficient fluorescent probe has been successfully applied to dynamic monitoring of endogenous and exogenous •OH and ClO- in live cells.

Colorimetric determination of tetracyclines based on aptamer-mediated dual regulation of gold nanoparticle aggregation and in situ silver metallization.

A colorimetric assay was described for determination of tetracyclines (TCs) in complex matrices based on dual regulation of gold nanoparticle (AuNP) aggregation and in situ silver metallization. The reaction between p-aminophenol and silver ions (Ag+) catalyzed by aptamer (Apt)-functionalized AuNPs promoted the in situ deposition of Ag shells on AuNPs to afford core-shell bimetallic NPs (Au@AgNPs). When the target is present, Apt is separated from the AuNP surface through the Apt-target interaction. With the desorption of Apts, AuNPs catalyzed the formation of more aggregated Au@AgNPs with a thinner Ag shell, corresponding to the solution color gradually changing from orange-yellow to brown. Using A600/A410 as the determination signal, the assay gave visible linear relationships for TCs in the range of 0.3-6.0 µmol L-1 with a detection limit (LOD) of 33.6 nmol L-1. In particular, the assay was successfully applied to detect TCs in real samples with the recovery rate ranging from 91.2% to 106% and relative standard deviation (RSD) less than 4.6%. These results meet the requirements of sample analysis.

Visualizing the degradation of nerve agent simulants using functionalized Zr-based MOFs: from solution to hydrogels.

Visual monitoring of the degradation of nerve agent simulants based on the switchable fluorescence of UiO-66-NH2 was developed. In the hydrolysis, the decomposition products perturbed the linker-to-cluster charge transfer and stimulated the fluorescence recovery. Moreover, a "soft" solid-state platform utilizing agarose hydrogels was proposed to visualize the degradation of gaseous simulants without bulk water.

A Galactosidase-Activatable Fluorescent Probe for Detection of Bacteria Based on BODIPY.

Pathogenic E. coli infection is one of the most widespread foodborne diseases, so the development of sensitive, reliable and easy operating detection tests is a key issue for food safety. Identifying bacteria with a fluorescent medium is more sensitive and faster than using chromogenic media. This study designed and synthesized a ß-galactosidase-activatable fluorescent probe BOD-Gal for the sensitive detection of E. coli. It employed a biocompatible and photostable 4,4-difluoro-3a,4a-diaza-s-indancene (BODIPY) as the fluorophore to form a ß-O-glycosidic bond with galactose, allowing the BOD-Gal to show significant on-off fluorescent signals for in vitro and in vivo bacterial detection. This work shows the potential for the use of a BODIPY based enzyme substrate for pathogen detection.

Rapid visual detection for nitroreductase based on the copper ions-induced and NADH-mediated aggregation of gold-silver alloy nanoparticles.

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 Au80Ag20 NPs caused a more prominent color change. Copper ions (Cu2+) can be rapidly reduced by nicotinamide adenine dinucleotide (NADH) under the catalysis of Au80Ag20 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 Au80Ag20 NPs by the excess Cu2+ and different aggregation degrees of Au80Ag20 NPs lead to observable color change. A linear correlation of A600/A505 = 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, Au80Ag20 NPs was replaced by several different composition nanoparticles (Au NPs, Au70Ag30 NPs, Au50Ag50 NPs) to be a chromogenic substrate, and the results suggest the Au80Ag20 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.

Polydopamine molecularly imprinted polymer coated on a biomimetic iron-based metal-organic framework for highly selective fluorescence detection of metronidazole.

Molecular imprinting technology was used to coat polydopamine (PDA) onto MIL-53(Fe) surface by simple self-polymerization. The MIL-53(Fe)@MIP composite with enhanced peroxidase-like activity and specific target recognition function was synthesized and selected to construct a fluorescence sensor to detect metronidazole (MNZ). Since the substrate terephthalic acid was incorporated in the framework of MIL-53(Fe)@MIP, no additional luminescent substrate was required. This avoided the interference of the substrate on the enzymatic detection system and improved the accuracy of the assay. The characteristics of MIL-53(Fe)@MIP composite were investigated and confirmed by systematic analyses. The experimental results proved that the sensor provided satisfactory performances for quantitative determination of MNZ in wide linear range from 1 to 200 µM with low limit of detection as 53.4 nM. Potential interfering substances such as common cations and anions, amino acids, other antibiotics, sugars, and food additive were studied to show negligible effect on the assay, allowing the practical application to different fields including milk and human serum by the standard addition method. The recoveries were obtained between 93.2 and 102%, and the RSD was less than 3%.

Colorimetric determination of acid phosphatase activity and inhibitor screening based on in situ polymerization of aniline catalyzed by gold nanoparticles.

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. Graphical abstract Schematic presentation of the colorimetric determination of acid phosphatase activity and inhibitor screening based on in situ polymerization of aniline catalyzed by gold nanoparticles. : acid phosphatase (ACP); : gold nanoparticles (AuNPs); : gold-polyaniline core-shell nanoparticles (Au@PANI NPs); ascorbic acid (AA); 2-phospho-L-ascorbic acid trisodium salt (AAP).

Tuning the excited-state intramolecular proton transfer (ESIPT)-based luminescence of metal-organic frameworks by metal nodes toward versatile photoluminescent applications.

Here, using three metal cations (Mg2+, Al3+, and Zr4+) and an excited-state intramolecular proton transfer (ESIPT) active linker, 2,5-dihydroxyterephthalic acid (H2DHT), three luminescent metal-organic frameworks (LMOFs) were obtained. Importantly, their ESIPT-based luminescence originated from the linker was systematically tuned in emission profiles including intensity, emission color, and quantum efficiency in the solution as well as in the solid state, which is largely dependent on the composition and structural characteristics of these three LMOFs. Similar to the free linker, the Mg-based MOF possesses a relatively strong luminescence, the Al-based MOF has moderate luminescence due to the breathing effect, and the Zr-based MOF is very weakly luminescent, mainly caused by the LMCT process. Benefiting from unique emission behaviors of these three LMOFs, we further modulated their ESIPT-based luminescence through the interplay between guest species and components of LMOFs by combining with various photophysical processes, and successfully explored their potential applications as versatile photoluminescent platforms for target-triggered sensory materials, responsive fluorescent hydrogels, and white-light-emitting phosphors.

A novel light-controlled colorimetric detection assay for nitroreductase based on p-aminophenol-catalyzed and NADH-mediated synthesis of silver nanoparticles.

A novel and efficient light-controlled colorimetric assay for the quantification and detection of nitroreductase (NTR) was constructed based on p-aminophenol (pAP)-catalyzed and nicotinamide adenine dinucleotide (NADH)-mediated generation of AgNPs. Due to the hydrolysis of p-nitrophenol by NTR in the presence of NADH, the hydrolysis product can be used as a catalyst to catalyze the reduction of Ag+ by NADH under the light. As the concentration of NTR increases, the value of absorbance at ca. 400 nm (A400) decreases and the color of the solution turns from brown to bright yellow. A linear correlation was obtained between A400 and the NTR concentration in the range from 1-50 µg mL-1 and the limit of detection (LOD) is 0.27 µg mL-1. The detection system does not respond to other common biological molecules due to the specificity of enzymes and the effect of the nitroreductase inhibitor on the NTR activity was also tested. Finally, we applied the assay to determine NTR in human serum samples by spiking different concentrations of NTR with a recovery of 85.2%-92.5%.

Concentration-dependent photoluminescence carbon dots for visual recognition and detection of three tetracyclines.

Concentration-dependent photoluminescence carbon dots (CDs) have been successfully synthesized through the one-step hydrothermal treatment of o-phthalic acid and ethylenediamine. The CDs possessed higher fluorescence quantum yield, up to 39.22%, exhibiting distinguished optical property, water solubility, and stability. The CDs that emit strong blue-green fluorescence can visually identify and determine tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC). TC quenched the fluorescence of CDs at 500 nm owing to the inner filter effect; OTC behaved similarly, but the emission wavelength of CDs was red-shifted to 515 nm. Inversely, once CTC was introduced to CDs solution, the fluorescence increased and the emission peak was blue-shifted to 450 nm. Bandgap transition and electrostatic interaction were proposed to be the mechanisms for the detection of OTC and CTC by CDs. Wide linear relationships were established for TC, OTC, and CTC with the limits of detection to be 50 nM, 36 nM, and 373 nM, respectively. Furthermore, the nanoscale probe constructed by this system has been applied to detect tetracyclines (TCs) in complex samples with satisfying recoveries (93.2-114%) and was designed as a portable test strip sensor for visually on-site TCs of honey sample screening. Accordingly, the preparation process of the nano fluorescent probe is simple and environmentally friendly, and the probe has a specific recognition ability for tetracyclines. The synthesized CDs in this work provide a new orientation for fast, effective, and visual real-time detection of tetracycline in actual samples.

Smartphone colorimetric assay of acid phosphatase based on a controlled iodine-mediated etching of gold nanorods.

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.

Colorimetric determination of tyrosinase based on in situ silver metallization catalyzed by gold nanoparticles.

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. Graphical abstract Schematic presentation of colorimetric assay for tyrosinase (TYRase) based on the inhibition effect on silver deposition onto catalytically active gold nanoparticles (AuNPs) and its application with a smartphone. Tyrosinase (TYRase); silver ions (Ag+); kojic acid (KA); gold nanoparticles (AuNPs); gold-silver core-shell nanoparticles (Au@Ag NPs).