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The global spread of SARS-CoV-2 virus has severely affected human health, life, and work. Vaccine immunization is considered to be an effective means to protect the body from infection. Therefore, timely analysis of the antibody level is helpful to identify people with low immune response or attenuated antibodies so as to carry out targeted and precise vaccine booster immunization. Herein, we develop a magnetofluid-integrated multicolor immunochip, as a sample-to-answer system in a fully enclosed space, for visual analysis of neutralizing antibodies of SARS-CoV-2. Generally, this chip adopts an innovative three-dimensional two-phase system that utilizes mineral oil to block the connection between reagent wells in the vertical direction and provides a wide interface for rapid and nondestructive shuttle of magnetic beads during the immunoassay. In order to obtain visualized signal output, gold nanorods with a size-dependent color effect are used as the colorful chromogenic substrates for evaluation of the antibody level. Using this chip, the neutralizing antibodies were successfully detected in vaccine-immunized volunteers with 83.3% sensitivity and 100% specificity. Furthermore, changes in antibody levels of the same individual over time were also reflected by the multicolor assay. Overall, benefiting from simple operation, airtight safety, and nonrequirement of external equipment, this platform can provide a new point-of-care testing strategy for alleviating the shortage of medical resources and promoting epidemic control in underdeveloped areas.

Photothermal biosensors with advantages of speed and high sensitivity offer alternative and reliable solutions for real-time clinical diagnosis, food safety testing and environmental monitoring. Although metallic nanoparticles are usually used for photothermal biosensors, their poor photothermal stability and potential toxicity hinder clinical applications. Taking advantage of the low cytotoxicity and remarkable photothermal effect under the low laser power of polypyrrole-based organic nanoparticles, we developed a novel photothermal biosensor with a temperature and pressure dual readout. After the formation of immunoassay sandwich structures, polypyrrole as the photothermal agent is synthesized

Colorimetric platform using the aggregation of gold nanoparticles (AuNPs) is a pretty simple method for biosensing, but advanced instruments such as specterophotometer is still needed to achieve accurately quantitative readout. Aggregated AuNPs exhibit excellent photothermal properties under near-infrared laser irradiation, which is significantly different from non-aggregated AuNPs. Herein, given the different photothermal effect, we translated the AuNPs-based colorimetric assay into a photothermal assay for the quantitative detection of adenosine using a thermometer as readout. Short single-stranded DNA (ssDNA, adenosine aptamer) was adsorbed on the surface of AuNPs and hence prevented the aggregation of AuNPs under high ionic concentration. The presence of adenosine caused the structural change of ssDNA and the AuNPs became aggregated. The enhanced temperature under NIR-laser irradiation has a linear response to the concentration of adenosine in the range of 2.0–50.0 μM. The detection limit was 1.7 μM. This proposed method is portable, easy and applicable to the quantitative assay of other targets by simply replacing of the sequence of ssDNA.

The dispersed gold nanoparticles (AuNPs) have weak photothermal effect in near-infrared (NIR) region. After the addition of cysteine, the AuNPs are aggregated due to the electrostatic interaction and then exhibited strong photothermal effect. At present of copper ion (Cu2+), the cysteine was catalytically oxidized into cystine, leading to the inhibition of the aggregation of AuNPs and the photothermal effect decreased. Based on this, a simple photothermal assay can be developed for Cu2+ detection using a common thermometer as readout. The change of the temperature (ΔT) of the system has a linear relationship with Cu2+ in the range of 10–300 nM with a detection limit of 7.4 nM (S/N = 3). Furthermore, through labeling the detection antibody in immunoassay with CuO nanoparticles as the source of Cu2+, a convenient photothermal immunoassay can be developed. Carcinoembryonic antigen (CEA), an important biomarker for cancer screening, was chosen as the model target because the rise of CEA level is widely present in cancer blood serum. Under the optimized conditions, ΔT has a linear relationship with CEA concentration in the range of 3.0–48.0 ng/mL. The detection limit is 1.3 ng/mL. The proposed method had been applied to detect CEA in serum samples with good agreement with the reference method used in hospital.

Hydrogen sulfide (H

To meet the increasing need for point-of-care testing (POCT), simple and portable readout strategies would be highly desirable. Thermometer with high accuracy and straightforward readout is an ideal tool for the development of new POCT methods. The exploration of new thermometer-based detection methods is of great significance. In this study, a simple biosensor for glucose based on the photothermal effect of gold nanorods using a simple thermometer as readout has been developed. In the presence of glucose oxidase, glucose can react with the dissolved oxygen to produce H2O2. With the help of Fe2+, H2O2 can etch gold nanorods (AuNRs) to different aspect ratios. The decrease of the aspect ratio of AuNRs leads to the blue-shift of the localized surface plasmon resonance peak, resulting in a decrease of photothermal effect in the near-infrared regions and the temperature of the system decreased. The change of the temperature has a linear relationship with the logarithm of glucose concentration in the range of 1.0–10.0 mM with a detection limit of 0.8 mM. The proposed method possesses a bias offset of −0.03 mM for glucose detection compared to the hospital method. Since many enzymatic reactions can produce H2O2, the principle can be modified to detect different targets by simply change of the enzyme used.

Microfluidics have attracted increasing attention, as they can precisely manipulate the fluid in a microscale channel or structure. Microfluidics-based biochemical sensing has been popularly used for various applications benefitting from the intrinsic advantages of low reagent consumption, rapid reaction rate, high throughput, easy integration, and facile automation. However, microfluidic device-based biochemical sensing is still limited by conventional signal readout that relies on sophisticated instruments and trained operators. Hence, the exploration of simplified signal readout is a critical issue in the development of microfluidic device-based biochemical sensing. In this review, equipment-free and low-cost signaling approaches for microfluidics are summarized, including colorimetric-readout, distance-readout, and smartphone-readout. Several other simplified signal readouts with potential to be integrated into microfluidics are described as well. Additionally, the prospects and challenges of simplified signal readout-based microfluidic devices are discussed.

Hydrogen sulfide (H2S), as a hazardous gas, is often found around dump areas. Long term exposure can cause harm to health, it is highly necessary to develop some simple and sensitive methods for on-site H2S detection. Herein, a convenient photothermal assay has been designed for the quantitation of H2S using a handheld thermometer as readout. Au@Ag nanocubes (Au@Ag NCs), a core-shell nanocomposite with strong light absorption at ∼450 nm, was chosen as a novel photothermal agent in this study. Under the laser irradiation at 450 nm, the Au@Ag NCs show a strong photothermal effect, and a significant temperature enhancement can be measured by the thermometer easily. The presence of H2S can lead to the deposition of sulfur onto Au@Ag NCs, altering the localized surface plasmon resonance absorption, size, surface composition, and morphology of Au@Ag NCs and hence leading to the reduction of photothermal effect. The change of the temperature has a linear relationship with the H2S concentration in the range of 0.5–80.0 μM with a detection limit of 0.35 μM. By combining with simple sample purification procedures, the developed method has been applied to detect H2S in garbage odor gas with satisfactory results.

A convenient photothermal biosensor was constructed for p53 DNA sequence detection based on the high discrimination capability of locked nucleic acid and high efficiency of signal amplification strategy of DNA walkers and difference photothermal effect between aggregated and dispersed gold nanoparticles (AuNPs). The presence of target activated the DNA walkers via the high affinity between target and complementary locked nucleic acid in the probe strand, resulting in the hybridization of the walker strand and substrate strand to form a specific enzyme recognition site. Under the cleavage of the endonuclease, single-stranded DNA (ssDNA) was released to the solution. Then the walker strand bound to a new substrate strand, and the next round of cleavage was triggered. The released ssDNA enhanced the stability of AuNPs against salt-induced aggregation. Given difference photothermal effects of the aggregated AuNPs and dispersed AuNPs under the near-infrared laser, the change of the temperature was detected by a common thermometer easily, which had a linear relationship with the target concentration in the range of 2.0–120.0 pM, the detection limit was 1.4 pM (S/N = 3). The proposed photothermal assay has been applied to detect p53 DNA sequence spiked complex samples with satisfying results.

We report a very easy and effective approach for synthesizing unique palladium-on-gold supra-nanostructure (Au@Pd-SprNS)-decorated graphene oxide (GO) nanosheets. The SprNSs comprising Au nanorods as core and a unique close-packed assembly of tiny anisotropic Pd nanoparticles (NPs) as shell were homogeneously distributed on the GO surface via electrostatic self-assembly. Compared with the traditional one-pot method for synthesis of metal NPs on GO sheets, the size and shape of core-shell Au@Pd SprNSs can be finely controlled and uniformly distributed on the GO carrier. Interestingly, this Au@Pd-SprNSs/GO nanocomposite displayed high electrocatalytic activities toward the oxidation of methanol, ethanol, and formic acid, which can be attributed to the abundance of intrinsic active sites including high density of atomic steps, ledges and kinks, Au-Pd heterojunctions and cooperative action of the two metals of the SprNSs. Additionally, uniform dispersion of the SprNSs over the GO nanosheets prevent agglomeration between the SprNSs, which is of great significance to enhance the long-term stability of catalyst. This work will introduce a highly efficient Pd-based nanoelectrocatalyst to be used in fuel cell application.

Ligand exchange on the surface of gold nanorods (AuNRs) is widely used, but conventional methods usually require multiple centrifugation cycles to completely remove cetyltrimethylammonium bromide (CTAB). This can lead to undesired aggregation of AuNRs. A dialysis-assisted protocol is described here for ligand exchange on AuNRs. Dialysis driven by a concentration gradient is shown to be a powerful tool to separate CTAB from aqueous solutions. The concentration gradient of CTAB in a dialysis bag can avoid the possible aggregation of AuNRs that can be caused by drastic environmental changes. It also supports the rate of ligand exchange on the surfaces of the AuNRs. The modified AuNRs were employed in a lateral-flow test strip immunoassay (LFTS-IAs) for the food pathogen E. coli O157:H7 in order to study of efficiency of ligand exchange. Compared to AuNRs where ligand exchange was performed via multiple centrifugation cycles, the AuNRs prepared by dialysis-assisted ligand exchange show improved conjugation to antibody and enhanced visual signals in the test line of the LFTS-IAs. A portable strip reader (absorption wavelength = 525 nm) is used to records the testing results. The sensitivity of AuNRs modified by dialysis has been achieved even as low as 1 × 102 cfu·mL−1 in a short time (within 15 min), and the working range is 1 × 102 to 1 × 106 cfu·mL−1, which is superior over the detection performance of conventional test strip using AuNRs modified by centrifugation.

An ultrasensitive and specific electrochemiluminescence (ECL) biosensor has been designed for the p53 DNA sequence, which is based on cascade signal amplification of nicking endonuclease assisted target recycling and hyperbranched rolling circle amplification (HRCA). First of all, biotin modified hairpin capture DNA (HP) probe was immobilized on the surface of streptavidin magnespheres paramagnetic particles (PMPs). Target DNA hybridized with the loop portion of the HP probe, therefore unfolding HP to form a double-stranded DNA (dsDNA) containing the specific nicking site of the nicking endonuclease. Then, the nicking endonuclease recognized the specific nicking site and cleaved the HP into two pieces, liberating target DNA and the complementary sequence piece for the padlock probe. The intact target DNA would initiate the next cycle of hybridization and cleavage, thereby releasing multiple complementary sequences for the padlock probes. The liberated complementary sequences hybridized with the padlock probes, subsequently inducing the HRCA reaction and generating numerous dsDNA segments. Herein, Ru(phen)3(2+) was embedded into dsDNA and worked as ECL signal reporter. The reaction products were eventually pretreated by dialysis tube with the cutoff membrane to remove the residual Ru(phen)3(2+) in the solution for the following ECL measurements. Using this cascade amplification strategy, an ultrasensitive p53 DNA sequence detection method was developed with a wide linear range from 0.05 to 100 fM and a low detection limit of 0.02 fM. Moreover, this cascade amplified ECL biosensor had specific recognition capacity for noncomplementary and single- and double-base mismatched DNA. The proposed ECL biosensor might have a great potential in biomedical research and clinic analysis.

Calcium lanthanide oxyborate doped with rare-earth ions LnCa(4)O(BO(3))(3):RE(3+) (LnCOB:RE, Ln = Y, La, Gd, RE = Eu, Tb, Dy, Cc) was synthesized by the method of solid-state reaction at high temperature. Their fluorescent spectra were measured from vacuum ultraviolet (VUV) to visible region at room temperature. Their excitation spectra all have a broadband center at about 188 nm, which is ascribed to host absorption. Using Dorenbos' and J phi rgensen's work [P. Dorenbos, J. Lumin. 91 (2000) 91, R. Resfeld, C.K. J phi rgensen. Lasers and Excite States of Rare Earth [M], Springer, Berlin, 1977, p. 45], the position of the lowest 5d levels E(Ln,A) and charge transfer band E(ct) were calculated and compared with their excitation spectra. Eu(3+) and Tb(3+) ions doped into LnCOB show efficient luminescence under VUV and UV irradiation. In this system, Ce(3+) ions do not show efficient luminescence and quench the luminescence of Tb(3+) ions when Tb(3+) and Ce(3+) ions are co-doped into LnCOB. GdCOB doped with Dy(3+) shows yellowish white light under irradiation of 254 nm light for the reason that Gd ions transfer the energy from itself to Dy. Because of the existence of Gd, the samples of GdCOB:RE(3+) show higher excitation efficiency than LaCOB:RE(3+) and YCOB:RE(3+) around 188 nm, which indicates that the Gd(3+) ions have an effect on the host absorption and can transfer the excitation energy to the luminescent center such as Tb(3+), Dy(3+) and Eu(3+). (c) 2005 Elsevier B.V. All rights reserved.

The gamma-Fe2O3 nanoparticles coated with DBS and CTAB were prepared by the microemulsion method. The coated samples show enhanced nonlinear optical properties compared with their bulk counterpart. The Mossbauer spectra at room temperature were measured for the coated ferric oxides. The monolayers of the organic molecules are found to have a strong chemical bond with the surface atoms and thus have a significant influence on the electron structure of particle surface. (C)1999 Acta Metallurgica Inc.