Nanozyme-based sensing of dopamine using cobalt-doped hydroxyapatite nanocomposite from waste bones

Dopamine is one of the most important neurotransmitters and plays a crucial role in various neurological, renal, and cardiovascular systems. However, the abnormal levels of dopamine mainly point to Parkinson's, Alzheimer's, cardiovascular diseases, etc. Hydroxyapatite (HAp), owing to its catalytic nature, nanoporous structure, easy synthesis, and biocompatibility, is a promising matrix material. These characteristics make HAp a material of choice for doping metals such as cobalt. The synthesized cobalt-doped hydroxyapatite (Co-HAp) was used as a colorimetric sensing platform for dopamine. The successful synthesis of the platform was confirmed by characterization with FTIR, SEM, EDX, XRD, TGA, etc. The platform demonstrated intrinsic peroxidase-like activity in the presence of H2O2, resulting in the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). The proposed sensor detected dopamine in a linear range of 0.9-35 μM, a limit of detection of 0.51 µM, limit of quantification of 1.7 µM, and an R2 of 0.993. The optimization of the proposed sensor was done with different parameters, such as the amount of mimic enzyme, H2O2, pH, TMB concentration, and time. The proposed sensor showed the best response at 5 mg of the mimic enzyme, pH 5, 12 mM TMB, and 8 mM H2O2, with a short response time of only 2 min. The fabricated platform was successfully applied to detect dopamine in physiological solutions.

Simple, sensitive, colorimetric detection of pyrophosphate via the analyte-triggered decomposition of metal-organic frameworks regulating their adaptive multi-color Tyndall effect

This paper describes initially the application of the Tyndall effect (TE) of metal-organic framework (MOF) materials as a colorimetric signaling strategy for the sensitive detection of pyrophosphate ion (PPi). The used MOF NH2-MIL-101(Fe) was prepared with Fe3+ ions and fluorescent ligands of 2-amino terephthalic acid (NH2-BDC). The fluorescence of NH2-BDC in MOF is quenched due to the ligand-to-metal charge transfer effect, while the NH2-MIL-101(Fe) suspension shows a strong TE. In the presence of PPi analyte, the MOFs will undergo decomposition because of the competitive binding of Fe3+ by PPi over NH2-BDC, resulting in a significant decrease in the TE signal and fluorescence restoration from the released ligands. The results demonstrate that the new method only requires a laser pointer pen (for TE creation) and a smartphone (for portable quantitative readout) to detect PPi in a linear concentration range of 1.25-800 μM, with a detection limit of ~210 nM (3σ) which is ~38 times lower than that obtained from traditional fluorescence with a spectrophotometer (linear concentration range, 50-800 µM; detection limit, 8.15 µM). Moreover, the acceptable recovery of PPi in several real samples (i.e., pond water, black tea, and human serum and urine) ranges from 97.66 to 119.15%.

Carbon dot/Co-MOF nanocoral mediated fluorescence-scattering ratiometric sensor for highly sensitive detection of alkaline phosphatase

Abnormal expression of alkaline phosphatase (ALP) in serum has received considerable attention in health monitoring and disease diagnosis. However, conventional optical analysis based on a single signal must compromise background interference and limited sensitivity in trace analysis. As an alternative candidate, the ratiometric approach depends on the self-calibration of two independent signals in a single test to minimize interferences from the background for accurate identification. Here, a carbon dot/cobalt-metal organic framework nanocoral (CD/Co-MOF NC) mediated fluorescence-scattering ratiometric sensor has been developed for simple, stable, and highly sensitive detection of ALP. ALP-responsive phosphate production was used to coordinate cobalt ion and collapse the CD/Co-MOF NC, resulting in the recovery of fluorescence signal from dissociative CDs and the decrease of second-order scattering (SOS) signal from the cracked CD/Co-MOF NC. The ligand-substituted reaction and the optical ratiometric signal transduction provide a rapid and reliable chemical sensing mechanism. The ratiometric sensor effectively converted ALP into a ratio signal of fluorescence-scattering dual emission throughout a wide linear concentration range of six orders of magnitude with a detection limit of 0.6 mU/L. In addition, self-calibration of fluorescence-scattering ratiometric method can reduce background interference and improve sensitivity in serum, approaching recoveries of ALP from 98.4% to 101.8%. Due to the above advantages, the CD/Co-MOF NC mediated fluorescence-scattering ratiometric sensor readily provides rapid and stable quantitative detection of ALP as a promising in vitro analytical method for clinical diagnostics.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Construction of Metal Organic Framework-Derived Fe-N-C Oxidase Nanozyme for Rapid and Sensitive Detection of Alkaline Phosphatase

Alkaline phosphatase (ALP) is a phosphomonoester hydrolase and serves as a biomarker in various diseases. However, current detection methods for ALP rely on bulky instruments, extended time, and complex operations, which are particularly challenging in resource-limited regions. Herein, we synthesized a MOF-derived Fe-N-C nanozyme to create biosensors for the coulometric and visual detection of ALP. Specifically, we found the Fe-N-C nanozyme can efficiently oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue-colored tetramethyl benzidine (TMBox) without the need for H2O2. To construct the biosensor, we incorporated the ALP enzymatic catalytic reaction to inhibit the oxidation of TMB by Fe-N-C oxidase nanozyme. This biosensor showed rapid and highly sensitive detection of ALP in both buffer and clinical samples. The limit of detection (LOD) of our approach could be achieved at 3.38 U L-1, and the linear range was from 5 to 60 U L-1. Moreover, we also developed a visual detection for ALP by using a smartphone-based assay and facilitated practical and accessible point-and-care testing (POCT) in resource-limited areas. The visual detection method also achieved a similar LOD of 2.12 U L-1 and a linear range of 5-60 U L-1. Our approach presents potential applications for other biomarker detections by using ALP-based ELISA methods.

Synergistic effect-triggered fluorescence quenching enables rapid and sensitive detection of alkaline phosphatase

The development of biosensors mediated by synergistic quenching effect is of great significance for rapid and accurate clinical diagnosis. Hence, we prepared a cyan-emitting fluorescent Si dots for alkaline phosphatase (ALP) detection through the synergistic quenching effect of inner filter effect (IFE) and photo-induced electron transfer (PET). Si dots were prepared by microwave-assisted method, which displayed high quantum yield (28.7%), as well as good physiochemical properties, such as photo-stability, pH stability, and chemical stability. As the hydrolysate of 4-nitrophenyl phosphate disodium salt hexahydrate catalyzed by ALP, both IFE and PET of 4-nitrophenyl to Si dots were used for the turn-off mode detection of ALP. The linear relationships were established between the change of fluorescence intensity and ALP concentration in the range of 0.05 U L^-1 to 5.0 U L-1, and 5.0 U L^-1 to 80.0 U L^-1, respectively. The detection limit was 0.01 U L^-1. The synergistic quenching effect caused the turn-off mode detection to be more sensitive, and it can also be used for the accurate detection of ALP in human serum, thereby showing great anti-interference ability in complex environments.

Detection of Periodontal Disease Marker with Geometrical Transformation of Ag Nanoplates

Alkaline phosphatase (ALP) and interleukin-1beta (IL-1β) are crucial salivary biomarkers for the diagnosis of periodontal disease that harms the periodontal tissue along with tooth loss. However, there has been no way of sensitive and portable detection of both biomarkers in saliva with multivariate signal readout. In this work, we design the multicolorimetric ALP and IL-1β sensing platform based on geometrical transformation of silver nanoplate transducer. By utilizing enzymatic activity of ALP that dephosphorylates p-aminophenol phosphate (p-APP) to p-aminophenol (p-AP), localized surface plasmon resonance properties of silver nanoplate vary with ALP and show a distinct color change from blue to yellow based on a controlled seed transformation from triangular to hexagonal, rounded pentagonal, and spherical shape. The multicolor sensor shows an ALP detection range of 0-25 U/L with a limit of detection (LOD) of 0.0011 U/L, which is the lowest range of LOD demonstrated to date for state-of-the-art ALP sensor. Furthermore, we integrate the sensor with the conventional ELISA to detect IL-1β for multicolor signaling and it exhibits a linear detection range of 0-250 pg/mL and an LOD of 0.066 pg/mL, which is 2 orders of magnitude lower than the monochromic conventional ELISA (LOD of 3.8 pg/mL). The ALP multicolor sensor shows high selectivity with a recovery of 100.9% in real human saliva proving its reliability and suitability for the readily accessible periodontal diagnosis with multivariate signal readout.

A Novel Electrochemical Sensing Strategy Based on Poly (3, 4-ethylenedioxythiophene): Polystyrene Sulfonate, AuNPs, and Ag+ for Highly Sensitive Detection of Alkaline Phosphatase

Alkaline phosphatase (ALP) is a crucial marker for the clinical analysis and detection of many diseases. In this study, an accurate signal amplification strategy was proposed for the sensing and quantification of alkaline phosphatase using poly (3, 4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), gold nanoparticles (AuNPs), and Ag⁺. Signal amplification was achieved by the modification of PEDOT:PSS and AuNPs on glassy carbon electrodes. Atomic force microscopy was performed to characterize the morphology of the modified nanomaterials. To detect ALP, 1-naphthyl phosphate (1-NP) was used as the substrate, and alkaline phosphatase catalyzed 1-NP into 1-naphthol (1-N), which resulted in the reduction of Ag⁺ to Ag⁰ on the surface of the modified electrode (AuNPs/PEDOT:PSS/GCE). The deposition of Ag drastically enhanced the detection signal. Differential pulse voltammograms of 1-N, which is the enzymatic product from the ALP reaction with 1-NP, were recorded. In the linear range of 0.1-120 U L^-1, a quantitative analysis of alkaline phosphatase was achieved, with high sensitivity and a low detection limit of 0.03 U L^-1. Stable, selective, and reproducible electrochemical sensors were designed. Moreover, the proposed electrochemical sensor exhibited a prominent sensing performance in the spiked diluted human serum. Thus, the sensor can be used in numerous applications in alkaline phosphatase or other analyte detection.

Detection of Pyrophosphate and Alkaline Phosphatase Activity Based on PolyT Single Stranded DNA - Copper Nanoclusters

The determination of pyrophosphate and alkaline phosphatase activity plays a significant role in medical diagnosis. In this work, a label-free "ON-OFF-ON" fluorescence strategy is developed for the analysis of pyrophosphate and alkaline phosphatase activity. Using PolyT single strand DNA as templates to synthesize fluorescent copper nanoparticles, the coordination effect of pyrophosphoric acid on Cu²⁺ inhibited the generation of fluorescence. Afterwards, the addition of alkaline phosphatase into hydrolyze pyrophosphoric acid resulted in the release of Cu^2+, whereby the fluorescence intensity could be recovered. Thereupon enhanced-sensitivity for alkaline phosphatase was obtained (0.1 mU/L), much better than previously reported methods. Meanwhile, it could be performed directly in homogeneous solution, which was very close to the actual activity level of alkaline phosphatase under physiological conditions. Likewise, satisfactory results were also obtained in specificity assessment, which demonstrated its potential application in clinical diagnosis. Notably, a new, sensitive, low-cost, short-time, and high-sensitivity platform for alkaline phosphatase detection was constructed, and the design of biosensor using DNA-templated Copper nanoclusters (CuNCs) was instructed in this study.

Brightness Analysis per Moving Particle: In Situ Analysis of Alkaline Phosphatase in Living Cells

In situ quantitative analysis of enzymes such as phosphatase is important to understand a number of involved biological processes ranging from various metabolisms to signal transduction and cellular regulation. In this paper, a novel in situ measurement strategy was proposed to detect alkaline phosphatase (ALP) activity in different locations within single living cells. The principle is based on the measurement of the resonance light scattering brightness ratio (SBR) per moving nanoparticle that forms in an ALP-related chemical reaction. In the method, a novel resonance light scattering correlation spectroscopy (RLSCS) system was developed using two lasers for illumination or two detection channels. Using the gold nanoparticles (AuNPs) as probes, the Au@Ag nanoparticles (Au@Ag NPs) formed due to the ALP-catalyzed hydrolysis of ascorbic acid 2-phosphate (AAP) and the subsequent reduction-deposition reaction of Ag ions that occurred on the AuNPs. The SBR value per moving particle was determined based on the obtained RLS intensity traces and RLSCS curves. The SBR value was found to be not influenced by the intracellular viscosity and size that was confirmed in the experiments. The linear relation between the SBR and ALP activity was established and applied to detect ALP activity and evaluate the inhibition of different drugs. Finally, the method was successfully used to in situ measure ALP activity within living cells. The method overcomes the shortcoming of conventional methods that lack quantitative analysis and are susceptible to intracellular viscosity.

Ready-to-Use Colorimetric Platform for Versatile Enzyme Assays through Copper Ion-Mediated Catalysis

A low cost and versatile colorimetric platform is developed for selective detections of various enzymes. Similar to peroxidases, free copper ion catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂ and turns TMB into a blue product. Bindings from ligands toward copper ions inhibit this catalysis. Enzymes catalyze the reactions of related substrates with generation or consumption the ligands for the binding and thus in turn alter the color changes as responses toward the enzymes. With suitable substrates, exemplary enzymes, including trypsin, acid phosphatase, and tyrosinase, can be sensitively measured, with limits of detection of 0.003 μg/mL, 0.004 U/L, and 0.02 U/mL, respectively. This platform is built with directly available reagents, and the signals can be obtained with inexpensive photometers or visual observations. The low cost and convenience make it suitable for cases where complicated instrumentations are not available, such as point-of-care testing.

A multicolor biosensor for alkaline phosphatase activity detection based on the peroxidase activity of copper nanoclusters and etching of gold nanorods

A multicolor biosensor for ALP activity has been developed based on the peroxidase activity of copper nanoclusters and etching of gold nanorods.

FeMoO4 nanospheres-based nanozymatic colorimetry for rapid and sensitive pyrophosphate detection

Assays of pyrophosphate ion (PPi) are of remarkable biochemical significance due to their vital roles in the bioenergetic and metabolic processes or as disease indicators. Colorimetry is popular in the field of biosensing and detection because of its simplicity, speed and cost-effectiveness, but there is a lack of a suitable colorimetric probe. Herein, a novel colorimetric sensing platform has been established for the detection of pyrophosphate based on the FeMoO₄-H₂O₂-3,3',5,5'-tetra-methylbenzidine (TMB) system. Compared with most previously reported iron-based nanozymes, the as-obtained FeMoO₄ nanospheres with a rough surface possessed a much superior peroxidase-like catalytic activity (V_max = 28.47 × 10^-8 M s^-1) and substrate affinity (K_m = 0.174 mM) toward H₂O₂ catalysis. Due to the Fe(II) and PPi reaction, the presence of PPi could specifically restore blue oxidized TMB to colorless TMB, which led to a decrease in UV absorption at 652 nm. The absorbance change is proportional to the PPi concentration, with a linear detection range (from 0.5 to 25 μM) and a low detection limit of 0.3 μM (S/N = 3). Accordingly, its excellent selectivity and high sensitivity made it a potential colorimetric sensor for PPi analysis in actual water samples.

Multicolor diagnosis of salivary alkaline phosphatase triggered by silver-coated gold nanobipyramids

Alkaline phosphatase (ALP) is one of the most versatile biomarkers for early detection of several diseases, such as oral carcinomas and periodontitis; therefore, great efforts have been dedicated for developing an ALP biosensor. Multicolor detection of ALP in saliva is ideal for a point-of-care diagnosis; however, this approach is very challenging since spectral responses over wavelengths of several tens of nanometers have thus far remained difficult to achieve. In this work, a colorimetric biosensor for ALP assay has been developed based on ALP affinity to dephosphorylate glucose phosphate into glucose, which has the affinity to deposit Ag nanoshells onto Au nanobipyramids with a multicolor response. This approach provides a blue shift of localized surface plasmon resonance (LSPR) as large as 190 nm corresponding to distinctive color changes, from yellowish brown to red based on the thickness of the formed Ag shell around the Au nanobipyramids. The change in the LSPR has been conducted for highly sensitive quantitative bioassay of ALP with a detectable multicolor change with linear dynamic range of 0.1-20 U/L and low limit of detection (LOD) of 0.085 U/L. Furthermore, the developed multicolor ALP biosensor exhibits high selectivity with high recovery of 98.6% demonstrating  its reliability and suitability for a point-of-care diagnosis.

A colorimetric alkaline phosphatase biosensor based on p-aminophenol-mediated growth of silver nanoparticles

Alkaline phosphatase (ALP) is an enzyme that catalyzes the dephosphorylation of proteins, nucleic acids, and biomolecules. It is a potential biomarker for diverse diseases such as breast cancer, osteopenia, and hepatobiliary. Herein, we developed a colorimetric sensor for the ALP assay based on its enzymatic activity to dephosphorylate the p-aminophenol phosphate (pAPP) into pAP. In a solution containing silver nanoparticles (AgNPs) and Ag⁺ ions prepared using a low concentration of NaBH₄, pAP mediates the growth of AgNPs by reducing the concentration of Ag⁺ ions to enhance the intensity of localized surface plasmon resonance as the pAPP cannot induce a reduction of the remaining Ag⁺ due to the masking of the hydroxyl with phosphate. The quantitative assay of the ALP was demonstrated via the colorimetric detection of the pAP-mediated growth of AgNPs in the presence of an ALP. The highly sensitive enzymatic growth of AgNPs provided a wider dynamic linear range of 0.5-225 U/L with a lower limit of detection of 0.24 U/L than that previously reported. The use of pAP resulted in excellent selectivity of the sensor for the ALP assay in human serum, yielding a high recovery rate and a high precision of 99.2 ± 1.5 % for the standard addition method.

Portable smartphone device-based multi-signal sensing system for on-site and visual determination of alkaline phosphatase in human serum

To provide the basis for clinical diagnosis in an emergency case, a portable smartphone device-based multi-signal sensing system for on-site determination of alkaline phosphatase (ALP) is introduced. In this system, cobalt hydroxide (CoOOH) nanoflakes can oxidize O-phenylenediamine (OPD) to produce 2,3-diaminophenazine (OxOPD), resulting in a strong fluorescence at 565 nm and an absorbance at 420 nm, respectively. The ascorbic acid 2-phosphate (AAP) can be hydrolyzed by alkaline phosphatase (ALP) to yield ascorbic acid (AA). Then, AA reduces the CoOOH nanoflakes to produce Co²⁺, and AA is oxidized to form dehydroascorbic acid (DHAA), thereby inhibiting the formation of OxOPD. The reaction product DHAA further combines with OPD to yield 3-(1,2-dihydroxyethyl)furo[3,4-b]quinoxalin-1(3H)-one (DFQ) accompanied by a strong fluorescence at 430 nm. Based on this, the fluorometric assay for ALP has a wide linear range from 0.8 to 190 U/L with a low detection limit of 0.16 U/L, and the colorimetric assay from 3 to 130 U/L with a detection limit of 1.94 U/L. Moreover, a portable smartphone sensing platform integrated with fluorescent and colorimetric signals was established for rapid determination of ALP without spectrometers. Recoveries of 97-104% for spiked samples and relative standard deviations (RSD) of less than 2% (n = 3) confirmed the feasibility of the developed platform in complicated samples, opening up new horizons for on-site evaluation in the biomedical field.

A turn-on luminescence probe based on amino-functionalized metal-organic frameworks for the selective detections of Cu2+, Pb2+ and pyrophosphate

A "turn-on" fluorescent probe based on amino-functionalized metal-organic frameworks (MOF-5-NH₂) is developed for the detection of Cu²⁺, Pb²⁺ and pyrophosphate (P₂O₇^4-, PPi). The fluorescence emission of fluorescent materials obtained by one-step synthesis is attributed to organic ligands. Cu²⁺ and Pb²⁺ coordinate with the amino group on the surface of the MOF-5-NH₂, which is ascribed to the host-guest electron transfer between analyte and probe, giving rise to the fluorescence quenching. After adding PPi, the intense affinity between Cu²⁺ and PPi remove Cu²⁺ from the MOF-5-NH₂, blocking of the electron transfer process, and the fluorescence can be recovered. The limit of detection is 0.057, 0.25 and 0.32 μmol L^-1 for the detection of Cu²⁺, Pb²⁺ and PPi, respectively. This turn-on mode based fluorescent probe shows preferable sensitivity and specificity to detect Cu²⁺, Pb²⁺ and PPi. These results demonstrate that the fluorescent MOF-5-NH₂ as a sensing platform displays remarkably performance.

A redox modulated fluorescence nanoplatform for the detection of alkaline phosphatase activity with fluorescent polydopamine nanoparticles

Herein, we simply synthesized intrinsic fluorescent polydopamine nanoparticles (PDA NPs) in sodium hydroxide solution (NaOH, pH 11), and constructed a new fluorescence nanoplatform for the detection of alkaline phosphatase (ALP) using PDA NPs as an effective signal reporter. The nanoplatform was constructed by the combination of enzymatic hydrolysis of ALP to the substrate l-ascorbic acid-2-phosphate (AA2P) and the chemical redox reaction between l-ascorbic acid (AA) and mercury ion (Hg2+). The fluorescence of PDA NPs could be effectively quenched by Hg2+ through the coordination effect between Hg2+ and the functional groups on the surface of PDA NPs. However, the quenching effect was greatly inhibited by the addition of AA into the solution. Based on this point, the activity of ALP could be monitored by hydrolysis of the substrate AA2P to AA and the fluorescence output of PDA NPs. The nanoplatform exhibited high sensitivity and desirable selectivity for ALP detection. With a wide linear range of 0 to 18 U L-1, a detection limit of 0.4 U L-1 was achieved using the developed nanosensor. The proposed method could not only be used to screen the inhibitor of ALP but also be used to detect ALP activity in human serum samples successfully. Moreover, the strategy can easily be expanded to determining other kinds of enzymes participating in AA-generation reactions.

Sensitive Fluorescence Assay for the Detection of Alkaline Phosphatase Based on a Cu2+-Thiamine System

The authors describe a novel, facile, and sensitive fluorometric strategy based on a Cu²⁺-thiamine (Cu²⁺-TH) system for the detection of alkaline phosphatase (ALP) activity and inhibition. The principle of the method is as follows. Under a basic conditions, TH, which does not exhibit a fluorescence signal, is oxidized into fluorescent thiochrome (TC) by Cu²⁺. Ascorbic acid 2-phosphate (AAP), which is the enzyme substrate, is hydrolyzed to produce ascorbic acid (AA) by ALP. The newly formed AA then reduces Cu²⁺ to Cu⁺, which prevents the oxidation of TH by Cu²⁺; as a result, the fluorescent signal becomes weaker. On the contrary, in the absence of ALP, AAP cannot reduce Cu²⁺; additions of Cu²⁺ and TH result in a dramatic increase of the fluorescent signal. The sensing strategy displays brilliant sensitivity with a detection limit of 0.08 U/L, and the detection is linear in the concentration range of 0.1 to 100 U/L. This approach was successfully applied to ALP activity in human serum samples, indicating that it is reliable and may be applied to the clinical diagnosis of ALP-related diseases.

A novel copper ion sensing fluorescent probe for fast detection of pyrophosphate and alkaline phosphatase

A Cu²⁺ sensing fluorescent probe is synthesized via a Mannich reaction and is applied in the fluorescence detection of pyrophosphate and alkaline phosphatase.

Molecule-gated surface chemistry of Pt nanoparticles for constructing activity-controllable nanozymes and a three-in-one sensor

Herein, a simple strategy for constructing activity-controllable nanozymes is proposed based on the glutathione (GSH)-gated surface chemistry of citrate-capped Pt nanoparticles (PtNPs). PtNPs have been shown to have oxidase-like activity that can effectively catalyze the oxidation of 3,3',5,5'-tertamethylbenzidine (TMB) by O2, resulting in a typical color reaction from colorless to blue. We found that GSH can inhibit the oxidase-like activity of PtNPs as a molecule-gated surface chemistry element, resulting in a dramatic decrease of the oxidation of TMB. The addition of copper ions (Cu2+) could oxidize GSH into glutathione disulfide (GSSG), resulting in the distinct suppression of GSH-modulated PtNP surface chemistry and oxidase-like activity inhibition, which further results in a significant acceleration of TMB oxidation and the obvious recovery of intense blue color. Furthermore, the color-based detection signal associated with the redox of TMB indicator here was found to show good fluorescence and a photothermal effect and exhibit sensitive and selective response toward the proposed molecule-gated surface chemistry and Cu2+ target. On the basis of this phenomenon, we successfully constructed a three-in-one sensor for Cu2+ with a triple signal readout, colorimetric, photothermal (temperature), and fluorescence, depending on the proposed in situ modulation method for PtNP catalysis. The applicability of the three-in-one sensor was also demonstrated by measuring Cu2+ in human serum with a standard addition method, and the results are of satisfactory accuracy as confirmed by ICP-MS measurements.

A novel alkaline phosphatase activity sensing strategy combining enhanced peroxidase-mimetic feature of sulfuration-engineered CoOx with electrostatic aggregation

Given alkaline phosphatase (ALP) takes part in the phosphorylation/dephosphorylation processes in the body, its activity is universally taken as an important indicator of many diseases, and thus developing reliable and efficient methods for ALP activity determination becomes quite important. Here, we propose a new sensing strategy for ALP activity by integrating the improved peroxidase-mimicking catalysis of sulfuration-engineered CoO_x with the hexametaphosphate ion (HMPi)-mediated electrostatic aggregation. After sulfuration engineering, the CoO_x composite coming from the pyrolysis of ZIF-67 exhibits enhanced peroxidase-mimetic catalytic ability to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to its oxide TMBox, offering a remarkable color change from colorless to mazarine; with the presence of HMPi, the rapid electrostatic assembly of negatively charged HMPi and positively charged TMBox leads to the aggregation of the latter, resulting in a color fading phenomenon; when ALP is added in advance to hydrolyze the HMPi mediator, the aggregation procedure is significantly suppressed, and such that the solution color can be recovered. Based on this principle, efficient determination of ALP activity was gained, giving a wide detection scope from 0.8 to 320 U/L and a detection limit as low as 0.38 U/L. Reliable analysis of the target in serum samples was also achieved, verifying the feasibility and practicability of our strategy in measuring ALP activity for clinical applications. Graphical abstract.

Enzyme activity-modulated etching of gold nanobipyramids@MnO2 nanoparticles for ALP assay using surface-enhanced Raman spectroscopy

The detection of enzyme activity can provide valuable insights into clinical diagnosis. Herein, we synthesize gold nanobipyramids@MnO2 nanoparticles (AMNS) as the surface-enhanced Raman spectroscopy (SERS) substrate for the first time and design a "turn-on" SERS strategy for the detection of enzyme activity without the need for a complicated SERS nanotag preparation process. In the presence of alkaline phosphatase (ALP), 2-phosphate-l-ascorbic acid trisodium salt (AAP) can be hydrolyzed to ascorbic acid (AA), which can etch the shell of AMNS by reducing MnO2 to Mn2+. The cracked MnO2 shell-caused electromagnetic field enhancement from AMNS can give rise to a significant increase in the Raman intensity of the adsorbed molecules (i.e., crystal violet, CV) on the surfaces of nanobipyramids. Thus, the ALP activity can be accurately quantified based on the MnO2 shell thickness dependent Raman signal output from CV. A linear dynamic range from 0.4 to 20 mU mL-1 with a detection limit of 0.04 mU mL-1 is achieved, which is more sensitive than other spectroscopic methods for ALP detection. Because of its advantages of sensitivity, convenience and versatility, this approach provides a new perspective to disease-related biomolecular detection in the future.

Aptamer-Functionalized DNA Nanostructures for Biological Applications

DNA nanostructures hold great promise for various applications due to their remarkable properties, including programmable assembly, nanometric positional precision, and dynamic structural control. The past few decades have seen the development of various kinds of DNA nanostructures that can be employed as useful tools in fields such as chemistry, materials, biology, and medicine. Aptamers are short single-stranded nucleic acids that bind to specific targets with excellent selectivity and high affinity and play critical roles in molecular recognition. Recently, many attempts have been made to integrate aptamers with DNA nanostructures for a range of biological applications. This review starts with an introduction to the features of aptamer-functionalized DNA nanostructures. The discussion then focuses on recent progress (particularly during the last five years) in the applications of these nanostructures in areas such as biosensing, bioimaging, cancer therapy, and biophysics. Finally, challenges involved in the practical application of aptamer-functionalized DNA nanostructures are discussed, and perspectives on future directions for research into and applications of aptamer-functionalized DNA nanostructures are provided.

Redox-modulated colorimetric detection of ascorbic acid and alkaline phosphatase activity with gold nanoparticles

Gold nanoparticles (AuNPs) exhibit characteristic absorption peaks in the ultraviolet visible region due to their special surface plasmon resonance effect. This characteristic absorption peak would change with the relative colour varying from wine red to orange-yellow upon sequential addition of ascorbic acid (AA) into the mixture of AuNPs and Ag(I). Similar observations also could be found when the hydrolysis product of sodium l-ascorbyl-2-phosphate with alkaline phosphatase (ALP) was used as an alternative to AA. Results of structure characterization confirmed that the phenomena were due to the reduction of Ag(I) to Ag(0) on the surface of AuNPs and the formation of core-shell AuNPs@Ag. Therefore, a colorimetric assay for rapid visual detection of AA and ALP based on redox-modulated silver deposition on AuNPs has been proposed. Under the optimal experimental conditions, the absorbance variation ΔA_{522 nm} /A_{370 nm} of AuNPs was proportional to the concentration of AA (5-60 μmol/L) and ALP (3-18 U/L) with the corresponding detection limit of 2.44 μmol/L for AA and 0.52 U/L for ALP. The assay showed excellent selectivity towards AA and ALP. Moreover, the assay has been applied to detect AA and ALP activity in real samples with satisfying results.

One-pot synthesis of luminol–gallium nanoassemblies and their peroxidase-mimetic activity for colorimetric detection of pyrophosphate

Luminol–Ga nanoassemblies exhibit peroxidase-mimetic activity. Colorimetric detection of PPi is developed owing to the formation of a complex between PPi and Ga³⁺.

Mimicking biological process to detect alkaline phosphatase activity using the vitamin B6 cofactor conjugated bovine serum albumin capped CdS quantum dots

This manuscript presents a novel bioanalytical approach for the selective ratiometric fluorescent sensing of enzymatic activity of the alkaline phosphatase (ALP) in the biological samples. The probe was designed by conjugating the pyridoxal 5'-phosphate (PLP) over the surface of bovine serum albumin (BSA) stabilized CdS quantum dots (QDs) through the interaction of free amine present in BSA with the aldehyde group of PLP. The conjugation of PLP quenched the emission of QDs. Upon addition of the ALP, the emission of QDs was restored due to the dephosphorylation and the conversion of the functionalized PLP in to pyridoxal. With this probe, the ALP activity can be detected down to 0.05 U/L and also successfully applied for the detection of ALP activity in biological samples such as human serum and plasma.

Facile colorimetric detection of alkaline phosphatase activity based on the target-induced valence state regulation of oxidase-mimicking Ce-based nanorods

Alkaline phosphatase (ALP) is widely recognized as a significant biomarker for lots of diseases. For this reason, developing effective and simple methods to monitor ALP activity is strongly necessary. Herein, we propose a novel strategy based on the target-induced valence state regulation of oxidase-mimicking Ce-based nanorods for ALP activity sensing. The mixed-valent Ce-based material (MVCM) with a relatively high Ce(iv)/Ce(iii) ratio can exhibit good oxidase-like activity to trigger the catalytic oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue TMBox in the presence of O₂, resulting in a notable chromogenic reaction. When ALP hydrolyzes ascorbic acid phosphate into ascorbic acid (AA), the formed AA induces the partial reduction of the MVCM to one with a low Ce(iv)/Ce(iii) ratio, which shows much less activity to trigger the chromogenic reaction. According to the above principle, a facile colorimetric assay was developed for ALP activity detection, providing a linear range of 0.5-25 U L^-1 and a limit of detection of 0.1 U L^-1. Besides, the proposed strategy could offer favorable selectivity for ALP activity determination. Accurate sensing of the target in serum was demonstrated by our assay as well, revealing its promise as a reliable tool for clinical diagnosis.

A review on emerging principles and strategies for colorimetric and fluorescent detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is a natural enzyme that is able to catalyze the dephosphorylation of phosphate esters. It participates in a great number of biological processes ranging from various metabolisms to signal transduction and cellular regulation. Since the abnormality of ALP activity in body is closely associated with many diseases, it has become an important biomarker for clinical diagnosis and treatment. Besides, it is often utilized in enzyme-linked immunosorbent assays. Given these demands, in the last few years considerable interest has been focused on exploring new materials and methods for ALP activity detection. In this review, we first made a clear classification on the principles that could be used for ALP activity determination. After that, emerging colorimetric and fluorescent strategies designed on the basis of these principles were systematically summarized. Finally, some perspectives on ALP activity analysis were discussed, hoping to inspire future efforts in the field.

Spectrophotometric determination of the activity of alkaline phosphatase and detection of its inhibitors by exploiting the pyrophosphate-accelerated oxidase-like activity of nanoceria

The oxidase-like activity of nanoceria is low. This limits its practical applications. It is demonstrated here that pyrophosphate ion (PPi) can improve the oxidase-like activity of nanoceria. Specifically, nanoceria catalyzes the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to give a blue product (oxTMB) with an absorption peak at 645 nm in the presence of PPi. If, however, alkaline phosphatase (ALP) is present, it will hydrolyze PPi, and this results in a decreased oxidase-like activity of nanoceria. Hence, less blue oxTMB willl be formed. On the other hand, if the ALP inhibitor Na₃VO₄ is added to the system, the oxidase-like activity of nanoceria is gradually restored. On the basis of the above results, a spectrophotometric method was developed for determination of the activity of ALP. It works in the 0.5 to 10 mU.mL^-1 activity range and has a 0.32 mU.mL^-1 detection limit. Na₃VO₄ causes a 50% ALP inhibition if present in 71 μM concentration. The assay was successfully applied to the determination of ALP in spiked human serum and gave good recoveries. Graphical abstract Schematic presentation of pyrophosphate (PPi)-induced acceleration of the oxidase-like activity of nanoceria (CeO₂) for determination of alkaline phosphatase enzyme (ALP) activity and its inhibitor NaVO₃.

An exonuclease-assisted fluorescence sensor for assaying alkaline phosphatase based on SYBR Green I

In this report, we propose a fast, reliable and convenient approach to determine the alkaline phosphatase (ALP) activity based on a label-free fluorescence strategy. Upon catalysis of ALP, dephosphorylated dsDNA hampers the λ exonuclease (λexo) cleavage, shows high affinity to SYBR Green I (SG I), resulting in a strong fluorescence emission peak at 520 nm. In the absence of ALP, the dsDNA with 5'-phosphoryl-termini could be employed as a substrate of λexo. After cleavage, a weak fluorescence emission peaks at 520 nm could be observed. The assay was both selective and sensitive, and the detection limit was found to be as low as 3 U/L. This method was utilized to evaluate Na₃VO₄ as ALP inhibitor. The method was successfully applied to the determination of the activity of ALP in spiked human serum samples.

Enzyme-triggeredin situformation of Ag nanoparticles with oxidase-mimicking activity for amplified detection of alkaline phosphatase activity

ALP-triggeredin situformation of Ag NPs with high oxidase-mimicking activity for colorimetric detection of alkaline phosphatase activity.

Colorimetric sensing of alkaline phosphatase and α-fetoprotein based on the photoinduced oxidase activity of fluorescein

A simple and inexpensive colorimetric assay for alkaline phosphatase (ALP) and α-fetoprotein (AFP) has been established by the hydrolysis of fluorescein diphosphate (FDP).

Unlocking the hidden talent of DNA: Unexpected catalytic activity for colorimetric assay of alkaline phosphatase

Carboxylic acids have been efficiently used to activate H₂O₂ to form even more potent oxidant-peroxy acids through enzyme-catalyzed processes. By employing acetic acid as the activator, herein we report for the first time that cofactor-free DNA displays unexpected activity in H₂O₂-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) under mild conditions. A series of 10-nt oligonucleotides were rationally designed with various combinations of double nucleotides including TG, AG, CG, TA and AC respectively, which demonstrates that the catalytic performance of DNA is highly dependent upon the sequence composition, strand length and continuous nucleotides. Inspired by phosphate-induced inhibition effects on the formation of peracetic acid, an ultrasensitive assay was well-established for monitoring alkaline phosphatase (ALP) on the basis of double terminal-phosphorylated G-rich oligonucleotides. Phosphorylated DNA not only serves as the substrate for ALP-catalyzed hydrolysis, but also acts as the enzyme-like catalyst for signal amplification. Quantitative determination of ALP is realized in a linear range from 0.05 to 15 mU/mL, resulting in the limit of detection of 0.01 mU/mL. The rapid and reliable test also has great potential in analyzing serum samples for practical disease diagnosis.