Label-free colorimetric sensing of ascorbic acid based on Fenton reaction with unmodified gold nanoparticle probes and multiple molecular logic gates

Accuracy improvement via novel ratiometry design in distance-based microfluidic paper based analytical device: instrument-free point of care testing

Developing accurate, precise, instrument-free, and point-of-need microfluidic paper-based devices is highly significant in clinical diagnosis and biomedical analysis. In the present work, a ratiometric distance-based microfluidic paper-based analytical device (R-DB-μPAD), along with a three-dimensional (3D) multifunctional connector (spacer), was designed to improve the accuracy and detection resolution analyses. Specifically, the novel R-DB-μPAD was used for the accurate and precise detection of ascorbic acid (AA) as a model analyte. In this design, two channels were fabricated as detection zones, with a 3D spacer located between the sampling and detection zones to improve the detection resolution by preventing the reagents mixing from overspreading between these zones. Two probes for AA were used: Fe³⁺ and 1,10-phenanthroline were deposited in the first channel, and oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) was added to the second channel. Accuracy improvement of this ratiometry-based design was achieved by enhancing the linearity range and reducing the volume dependency of the output signal. Moreover, the 3D connector improved the detection resolution by eliminating the systematic errors. Under the optimal conditions, the ratio of the distances of the color bands in the two channels was used to construct an analytical calibration curve in the range from 0.05 to 1.2 mM, with a limit of detection of 16 μM. The proposed R-DB-μPAD combined with the connector was successfully used for the detection of AA in orange juice and vitamin C tablets with satisfactory accuracy and precision. This work opens the door for multiplex analysis of various analytes in different matrices.

Colorimetric sensors and nanoprobes for characterizing antioxidant and energetic substances

The development of analytical techniques for antioxidant compounds is important, because antioxidants that can inactivate reactive species and radicals are health-beneficial compounds, also used in the preservation of food and protection of almost every kind of organic substance from oxidation. Energetic substances include explosives, pyrotechnics, propellants and fuels, and their determination at bulk/trace levels is important for the safety and well-being of modern societies exposed to various security threats. Most of the time, in field/on site detection of these important analytes necessitates the use of colorimetric sensors and probes enabling naked-eye detection, or low-cost and easy-to-use fluorometric sensors. The use of nanosensors brings important advantages to this field of analytical chemistry due to their various physico-chemical advantages of increased surface area, surface plasmon resonance absorption of noble metal nanoparticles, and superior enzyme-mimic catalytic properties. Thus, this critical review focuses on the design strategies for colorimetric sensors and nanoprobes in characterizing antioxidant and energetic substances. In this regard, the main themes and properties in optical sensor design are defined and classified. Nanomaterial-based optical sensors/probes are discussed with respect to their mechanisms of operation, namely formation and growth of noble metal nanoparticles, their aggregation and disaggregation, displacement of active constituents by complexation or electrostatic interaction, miscellaneous mechanisms, and the choice of metallic oxide nanoparticles taking part in such formulations.

Gold nanozyme as an excellent co-catalyst for enhancing the performance of a colorimetric and photothermal bioassay

Advanced oxidation processes (AOPs) have recently proposed for advancing colorimetric sensing applications, owing to their excellent performance of sensitive color readout that generated from the oxidation of chromogenic substrates like 3,3',5,5'-tetramethylbenzidine (TMB) by reactive oxygen species (ROS) of AOPs such as ·OH and ·O₂^- radicals. However, the efficiency of ROS generation and the related H₂O₂ decomposition in most AOPs is quite low especially at neutral pH, which greatly hampered the practical sensing applications of the AOPs. We herein communicated that β-cyclodextrin (β-CD)-capped gold nanoparticles (β-CD@AuNPs) can promote catalysis at neutral pH for AOP as an excellent co-catalyst. In this strategy, inorganic pyrophosphate (PPi) ions was first used to coordinate with Cu²⁺ and form Cu²⁺-PPi complex. In the presence of hydrogen peroxide, target inorganic pyrophosphatase (PPase) can hydrolyze PPi into inorganic phosphate (Pi) and release free Cu²⁺ simultaneously, resulting in a Cu²⁺-triggered Fenton-like AOP reaction. The introduced β-CD@AuNPs acts as a co-catalyst, analogous to mediators in the most co-catalyzed system, to enhance the rate-limiting step of Cu²⁺/Cu⁺ conversion in Cu²⁺/H₂O₂ Fenton-like AOP and resulting in an efficient generation of ·OH and ·O₂^- radicals, which further producing an intense blue color by oxidizing TMB into its oxidation product (TMBox) within a short time. Finally, this reaction system was used to simply detecting target PPase with the colorimetric and photothermal readout based on the in-situ generated TMBox indicator. More significantly, we successfully demonstrated nanozyme can serve as a co-catalyst to promote the AOP catalysis at neutral pH, and inspire other strategies to overcome the pH limitation in the AOP catalysis and expand its colorimetric and photothermometric application.

Fabrication of Co3O4 NPs-graphene oxide nanocomposites as an efficient catalyst towards oxygen reduction and its catalytic applications

Co₃O₄ nanoparticles-graphene oxide (Co₃O₄ NPs-GO) nanocomposites with good solubility were successfully synthesized and well characterized. As a nanocatalyst with oxidase-mimicking activity, the nanocomposites can catalyze the oxidation of 3',5,5'-tetramethylbenzidine (TMB) with high efficiency. The catalyzing reaction was rapid and no extra H₂O₂ was needed compared with other similar TMB oxidation reactions. The catalyzing reaction mechanism of the system was investigated in detail. And it was demonstrated that the oxygen involved in the reaction came from the oxygen absorbed on the nanocomposites which oxidized TMB. Based on this reaction, a colorimetric system for vitamin C detection in vegetable and fruits was established. Under the optimum conditions, the detection can be achieved within 10 min and a linear relationship in concentration range of 2.5 × 10^-6 to 1.8 × 10^-5 M and 3.4 × 10^-5 to 1.7 × 10^-4 M was obtained with a detection limit of 7.0 × 10^-7 M. The colorimetric system exhibited good selectivity, sensitivity and satisfactory recoveries ranged from 93.1% to 101.1%.

Nanoscale fluorescent metal-organic framework composites as a logic platform for potential diagnosis of asthma

Asthma is a common chronic disorder, and the decreased hydrogen sulfide (H₂S) production in the lung has been considered as an early detection biomarker for asthma. However, the detection of H₂S in biological systems remains a challenge; because it requires the designed sensors to have the following features: nanoscale size, good biocompatibility, real-time detection, high selectivity/sensitivity, and good water stability. Here, we propose the potential of using nanoscale fluorescent metal-organic framework (MOF) composites Eu³⁺/Ag⁺@UiO-66-(COOH)₂ (hereafter denoted as EAUC) as a logic platform for tentative diagnosis of asthma by detecting the biomarker H₂S. This INHIBIT logic gate based on Eu³⁺@UiO-66-(COOH)₂ (EUC) can be produced by choosing Ag⁺ and H₂S as inputs and by monitoring the fluorescent signal (I₆₁₅) as an output. Our fluorescent studies indicate that the EAUC exhibits excellent selectivity, extreme sensitivity (limit of detection: 23.53 μM), and real-time in situ detection of H₂S. Further, MTT analysis in PC12 cells shows that the EAUC possesses low cytotoxicity and favourable biocompatibility that are suitable for the detection of biomarker H₂S in vivo, as demonstrated by the successful detection of spiked H₂S in the diluted serum samples. This work represents the possibility of using MOF-based logic platform for tentative diagnosis of asthma in clinical medicine.

Reproducible mesoporous silica-coated gold@silver nanoprobes for the bright colorimetric sensing of ascorbic acid

Herein, a colorimetric approach for the detection of ascorbic acid (AA) was developed by controlling the surface chemistry of silica-coated gold nanorod@silver nanoparticles (AuNR@Ag@mSiO₂).

A turn-on fluorescent probe for sensitive detection of ascorbic acid based on SiNP–MnO2nanocomposites

A nanoprobe prepared by coupling nanoparticles (SiNPs) with BSA templated-MnO₂nanosheets was constructed for ascorbic acid analysis.

Visual colorimetric 'turn-off' biosensor for ascorbic acid detection based on hypochlorite-3,3',5,5',-Tetramethylbenzidine system

An 'turn-off' approach for the detection of ascorbic acid (AA) using 3,3',5,5',-tetramethylbenzidine (TMB) as a colorimetric probe is developed. The proposed method is based on the fact that hypochlorite (ClO^-) could oxidize TMB to oxidized TMB (oxTMB) along with a blue color and an absorption peak centered at 652 nm. However, the introduction of AA could cause the reduction of oxTMB, which results in a blue color fading and a decrease of the absorbance at 652 nm. Based on this finding, we proposed a sensitive colorimetric assay for quantitative determination of AA by UV-vis spectroscopy. Under the optimal conditions, a good linear response between the absorbance and AA concentration was achieved within the dynamic working range from 1 to 70 μM with a detection limit of 0.58 μM. More importantly, the proposed method offered advantages of simple, low-cost instruments and rapid assay without the utilization of nanomaterials and has also been successfully applied for the determination of AA in orange juice with satisfying recoveries over 97.0%. Furthermore, the obtained results were consistent with those measured by high performance liquid chromatography with the average relative error of 3.3%.

A sensitive colorimetric assay system for nucleic acid detection based on isothermal signal amplification technology

Rapid and accurate detection of microRNAs in biological systems is of great importance. Here, we report the development of a visual colorimetric assay which possesses the high amplification capabilities and high selectivity of the rolling circle amplification (RCA) method and the simplicity and convenience of gold nanoparticles used as a signal indicator. The designed padlock probe recognizes the target miRNA and is circularized, and then acts as the template to extend the target miRNA into a long single-stranded nucleotide chain of many tandem repeats of nucleotide sequences. Next, the RCA product is hybridized with oligonucleotides tagged onto gold nanoparticles. This interaction leads to the aggregation of gold nanoparticles, and the color of the system changes from wine red to dark blue according to the abundance of miRNA. A linear correlation between fluorescence and target oligonucleotide content was obtained in the range 0.3-300 pM, along with a detection limit of 0.13 pM (n = 7) and a RSD of 3.9% (30 pM, n = 9). The present approach provides a simple, rapid, and accurate visual colorimetric assay that allows sensitive biodetection and bioanalysis of DNA and RNA nucleotides of interest in biologically important samples. Graphical abstract The colorimetric assay system for analyzing target oligonucleotides.

Near Infrared Graphene Quantum Dots-Based Two-Photon Nanoprobe for Direct Bioimaging of Endogenous Ascorbic Acid in Living Cells

Ascorbic acid (AA), as one of the most important vitamins, participates in various physiological reactions in the human body and is implicated with many diseases. Therefore, the development of effective methods for monitoring the AA level in living systems is of great significance. Up to date, various technologies have been developed for the detection of AA. However, few methods can realize the direct detection of endogenous AA in living cells. In this work, we for the first time reported that near-infrared (NIR) graphene quantum dots (GQD) possessed good two-photon fluorescence properties with a NIR emission at 660 nm upon exciting with 810 nm femtosecond pulses and a two-photon (TP) excitation action cross-section (δΦ) of 25.12 GM. They were then employed to construct a TP nanoprobe for detection and bioimaging of endogenous AA in living cells. In this nanosystem, NIR GQDs (NGs), which exhibited lower fluorescence background in living system to afford improved fluorescence imaging resolution, were acted as fluorescence reporters. Also CoOOH nanoflakes were chosen as fluorescence quenchers by forming on the surface of NGs. Once AA was introduced, CoOOH was reduced to Co²⁺, which resulted in a "turn-on" fluorescence signal of NGs. The proposed nanoprobe demonstrated high sensitivity toward AA, with the observed LOD of 270 nM. It also showed high selectivity to AA with excellent photostability. Moreover, the nanoprobe was successfully used for TP imaging of endogenous AA in living cells as well as deep tissue imaging.

Chemiluminescence determination of ascorbic acid using graphene oxide@copper-based metal–organic frameworks as a catalyst

In this work, composites with different amounts of graphene oxide (GO) and the copper-based metal–organic frameworks (HKUST-1) were synthesized.

Nanoparticles Affect PCR Primarily via Surface Interactions with PCR Components: Using Amino-Modified Silica-Coated Magnetic Nanoparticles as a Main Model

Nanomaterials have been widely reported to affect the polymerase chain reaction (PCR). However, many studies in which these effects were observed were not comprehensive, and many of the proposed mechanisms have been primarily speculative. In this work, we used amino-modified silica-coated magnetic nanoparticles (ASMNPs, which can be collected very easily using an external magnetic field) as a model and compared them with gold nanoparticles (AuNPs, which have been studied extensively) to reveal the mechanisms by which nanoparticles affect PCR. We found that nanoparticles affect PCR primarily by binding to PCR components: (1) inhibition, (2) specifity, and (3) efficiency and yield of PCR are impacted. (1) Excess nanomaterials inhibit PCR by adsorbing to DNA polymerase, Mg(2+), oligonucleotide primers, or DNA templates. Nanoparticle surface-active groups are particularly important to this effect. (2, a) Nanomaterials do not inhibit nonspecific amplification products caused by false priming as previously surmised. It was shown that relatively low concentrations of nanoparticles inhibited the amplification of long amplicons, and increasing the amount of nanoparticles inhibited the amplification of short amplicons. This concentration phenomenon appears to be the result of the formation of "joints" upon the adsorption of ASMNPs to DNA templates. (b) Nanomaterials are able to inhibit nonspecific amplification products due to incomplete amplification by preferably adsorbing single-stranded incomplete amplification products. (3) Some types of nanomaterials, such as AuNPs, enhance the efficiency and yield of PCR because these types of nanoparticles can adsorb to single-stranded DNA more strongly than to double-stranded DNA. This behavior assists in the rapid and thorough denaturation of double-stranded DNA templates. Therefore, the interaction between the surface of nanoparticles and PCR components is sufficient to explain most of the effects of nanoparticles on PCR.

Synthesis and properties of temperature-sensitive and chemically crosslinkable poly(ether-urethane) hydrogel

The PEU-MA solutions can gelate at physiological temperature, and be further crosslinked by UV light.

Cationic surfactant-based colorimetric detection of Plasmodium lactate dehydrogenase, a biomarker for malaria, using the specific DNA aptamer

A simple, sensitive, and selective colorimetric biosensor for the detection of the malarial biomarkers Plasmodium vivax lactate dehydrogenase (PvLDH) and Plasmodium falciparum LDH (PfLDH) was demonstrated using the pL1 aptamer as the recognition element and gold nanoparticles (AuNPs) as probes. The proposed method is based on the aggregation of AuNPs using hexadecyltrimethylammonium bromide (CTAB). The AuNPs exhibited a sensitive color change from red to blue, which could be seen directly with the naked eye and was monitored using UV-visible absorption spectroscopy and transmission electron microscopy (TEM). The reaction conditions were optimized to obtain the maximum color intensity. PvLDH and PfLDH were discernible with a detection limit of 1.25 pM and 2.94 pM, respectively. The applicability of the proposed biosensor was also examined in commercially available human serum.

Colorimetric detection of melamine in milk by citrate-stabilized gold nanoparticles

Here, we report a simple and sensitive colorimetric method for detection of melamine in milk using gold nanoparticles (AuNPs). AuNPs of 21-nm size were synthesized by the citrate reduction method. The method is based on the principle that the melamine causes the aggregation of AuNPs and, hence, the wine red color of AuNPs changes to blue or purple. This change in color can be visualized with the naked eye or an ultraviolet-visible (UV-Vis) spectrometer. Under optimized conditions, AuNPs are highly specific for melamine and can detect melamine down to a concentration of 0.05 mg L(-1).

Unmodified gold nanoparticles as a simple colorimetric probe for ramoplanin detection

In this paper unmodified gold nanoparticles (AuNPs) were used as a sensing element to detect ramoplamin. Detection relies on the fact that the dispersed AuNPs solution is red due to the intense surface plasmon absorption band at 530 nm whereas the AuNPs solution in the presence of ramoplanin is blue. Upon aggregation, there is a significant change in absorbance intensity at 620 nm. Based on the aggregation of AuNPs induced by ramoplanin, a simple colorimetric method was developed for determining the of ramoplanin concentration. Experimental conditions influencing the analytical performance such as particle size, amount of AuNPs, incubation time and pH were evaluated. Under the optimized experimental conditions, this method could detect ramoplanin in a linear range from 0.30 to 1.30 ppm with a detection limit of 0.01 ppm and exhibited good reproducibility, selectivity and recovery. Analysis time of this assay was only 2 min. To investigate its potential applicability, this assay was successfully applied for the determination of ramoplanin in urine samples without costly instruments.