Surfen-Assembled Graphene Oxide for Fluorescence Turn-On Detection of Sulfated Glycosaminoglycans in Biological Matrix

Sulfated glycosaminoglycans (GAGs) not only serve as a biomarker for mucopolysaccharidoses disease but also participate in various biological processes, such as blood clot medication (heparin) and signal transduction (heparan sulfate). However, few fluorescent sensors, such as 1,9-dimethylmethylene blue, have been developed for the detection of sulfated GAGs in the real world. Herein, we fabricated a surfen/few-layer graphene oxide (FLGO) nanocomplex for sensing sulfated GAGs in biological fluids. Surfen molecules are self-assembled onto the surface of FLGO through electrostatic attraction, and their fluorescence was then quenched by the creation of the FLGO-surfen complex (static quenching) and partially combined with the energy transfer from surfen to FLGO (dynamic quenching). The presence of sulfated GAGs resulted in the fluorescence recovery through the formation of the surfen-GAGs complex, which exhibits weak binding to FLGO and keeps surfen molecules away from the FLGO surface. Because FLGO efficiently reduced the fluorescence background from surfen and competed with sulfated GAGs for binding to surfen, surfen-assembled FLGO exhibited higher sensitivity and better selectivity for sulfated GAGs than surfen. The strategy mentioned above was exemplified by the analysis of heparin in human plasma and sulfated GAGs in an artificial cerebrospinal fluid; the limits of detection at a signal-to-noise ratio of 3 for heparin, dermatan sulfate, and heparin sulfate were determined to be 30, 30, and 60 ng/mL, respectively

1,4-Benzenediboronic-Acid-Induced Aggregation of Gold Nanoparticles: Application to Hydrogen Peroxide Detection and Biotin–Avidin-Mediated Immunoassay with Naked-Eye Detection

Hydrogen-peroxide (H₂O₂)-induced growth of small-sized gold nanoparticles (AuNPs) is often implemented for H₂O₂ sensing and plasmonic immunoassay. In contrast, there is little-to-no information in the literature regarding the application of H₂O₂-inhibited aggregation of citrate-capped AuNPs. This study discloses that benzene-1,4-diboronic acid (BDBA) was effective in driving the aggregation of citrate-capped AuNPs through an interaction between α-hydroxycarboxylate of citrate and boronic acids of BDBA. The H₂O₂-mediated oxidation of BDBA resulted in the conversion of boronic acid groups to phenol groups. The oxidized BDBA was incapable of triggering the aggregation of citrate-capped AuNPs. Thus, the presence of H₂O₂ prohibited BDBA-induced aggregation of citrate-capped AuNPs. The BDBA-induced aggregation of citrate-capped AuNPs can be paired with the glucose oxidase (GOx)–glucose system to design a colorimetric probe for glucose. Moreover, a H₂O₂·BDBA·AuNP probe was integrated with sandwich immunoassay, biotinylated antibody, and avidin-conjugated GOx for the selective naked-eye detection of rabbit immunoglobulin G (IgG) and human-prostate-specific antigen (PSA). The lowest detectable concentrations of rabbit IgG and human PSA by the naked eye were down to 0.1 and 4 ng/mL, respectively. More importantly, the proposed plasmonic immunoassay allowed the naked-eye quantification of 0–10 ng/mL PSA at an interval of 2 ng/mL in plasma samples

Self-Assembly of Monodisperse Carbon Dots into High-Brightness Nanoaggregates for Cellular Uptake Imaging and Iron(III) Sensing

This study describes a bottom-up assembly route for monodisperse carbon dots (CDs) into different sizes of CD aggregates through the control of the concentration of fatty acids. The highly monodisperse CDs were prepared via solvent–thermal treatment of edible soybean oil, which generated glycerol-based polymer as a carbon source and fatty acid as a surface capping in the synthetic process. The as-synthesized CDs exhibited small particle size variation (2.7 ± 0.2 nm) and narrow emission bands (full width at half-maximum <20 nm). The monodisperse CDs can self-assemble into blue-, green-, yellow-, and red-emitting CD aggregates by tuning the concentration of fatty acids. Compared to commercially available organic dyes and semiconductor quantum dots, the CD aggregates provided a 10–7000-fold improvement in brightness. Additionally, their emission wavelength was tunable across the entire visible spectrum by tuning the excitation wavelength. Because of their high brightness, fluorescence imaging of a single carbon dot and CD aggregate was simply achieved using filter-free dark-field fluorescence microscopy (DFM). We also demonstrate the use of filter-free DFM to dynamically image cellular uptake of the monodisperse CDs in MCF-7 cells and Huh-7 liver cancer cells. Without the conjugation of the fluorophore to the CDs, the particle aggregation-induced red-shifted emission enables the development of the CD-based ratiometric sensor for Fe^III ions and pyrophosphate based on Fe^III-induced aggregation of the monodisperse CDs

UV-Light-Induced Improvement of Fluorescence Quantum Yield of DNA-Templated Gold Nanoclusters: Application to Ratiometric Fluorescent Sensing of Nucleic Acids

The use of DNA as a template has been demonstrated as an effective method for synthesizing different-sized silver nanoclusters. Although DNA-templated silver nanoclusters show outstanding performance as fluorescent probes for chemical sensing and cellular imaging, the synthesis of DNA-stabilized gold nanoclusters (AuNCs) with high fluorescence intensity remains a challenge. Here a facile, reproducible, scalable, NaBH₄-free, UV-light-assisted method was developed to prepare AuNCs using repeats of 30 adenosine nucleotides (A₃₀). The maximal fluorescence of A₃₀-stabilized AuNCs appeared at 475 nm with moderate quantum yield, two fluorescence lifetimes, and a small amount of Au⁺ on the surface of the Au core. Results of size-exclusion chromatography revealed that A₃₀-stabilized AuNCs were more compact than A₃₀. A series of control experiments showed that UV light played a dual role in the reduction of gold-ion precursors and the decomposition of citrate ions. A₃₀ also acted as a stabilizer to prevent the aggregation of AuNCs. In addition, single-stranded DNA (ssDNA) consisting of an AuNC-nucleation sequence and a hybridization sequence was utilized to develop a AuNC-based ratiometric fluorescent probe in the presence of the double-strand-chelating dye SYBR Green I (SG). Under conditions of single-wavelength excitation, the combination of AuNC/SG-bearing ssDNA and perfectly matched DNA emitted fluorescence at 475 and 525 nm, respectively. The formed AuNC/SG-bearing ssDNA enabled the sensitive, selective, and ratiometric detection of specific nucleic acid targets. Finally, the AuNC-based ratiometric probes were successfully applied to determine specific nucleic acid targets in human serum

Amplified Peroxidase-Like Activity in Iron Oxide Nanoparticles Using Adenosine Monophosphate: Application to Urinary Protein Sensing

Numerous compounds such as protein and double-stranded DNA have been shown to efficiently inhibit intrinsic peroxidase-mimic activity in Fe₃O₄ nanoparticles (NP) and other related nanomaterials. However, only a few studies have focused on finding new compounds for enhancing the catalytic activity of Fe₃O₄ NP-related nanomaterials. Herein, phosphate containing adenosine analogs are reported to enhance the oxidation reaction of hydrogen peroxide (H₂O₂) and amplex ultrared (AU) for improving the peroxidase-like activity in Fe₃O₄ NPs. This enhancement is suggested to be a result of the binding of adenosine analogs to Fe²⁺/Fe³⁺ sites on the NP surface and from adenosine 5′-monophosphate (AMP) acting as the distal histidine residue of horseradish peroxidase for activating H₂O₂. Phosphate containing adenosine analogs revealed the following trend for the enhanced activity of Fe₃O₄ NPs: AMP > adenosine 5′-diphosphate > adenosine 5′-triphosphate. The peroxidase-like activity in the Fe₃O₄ NPs progressively increased with increasing AMP concentration and polyadenosine length. The Michaelis constant for AMP attached Fe₃O₄ NPs is 5.3-fold lower and the maximum velocity is 2.7-fold higher than those of the bare Fe₃O₄ NPs. Furthermore, on the basis of AMP promoted peroxidase mimicking activity in the Fe₃O₄ NPs and the adsorption of protein on the NP surface, a selective fluorescent turn-off system for the detection of urinary protein is developed

Oligonucleotide-Based Fluorescent Probe for Sensing of Cyclic Diadenylate Monophosphate in Bacteria and Diadenosine Polyphosphates in Human Tears

Cyclic diadenylate monophosphate (c-di-AMP) and P¹,P⁵-diadenosine-5′ pentaphosphate (Ap5A) have been determined to play important roles in bacterial physiological processes and human metabolism, respectively. However, few, if any, methods have been developed that use fluorescent sensors to sense c-di-AMP and Ap5A in the real world. To address this challenge, this study presents a fast, convenient, selective, and sensitive assay for quantifying c-di-AMP and Ap5A fluorescence based on the competitive binding of diadenosine nucleotides and a polyadenosine probe to coralyne. The designed probe consists of a 20-mer adenosine base (A₂₀), a fluorophore unit at the 5′-end, and a quencher unit at the 3′-end. Through A₂–coralyne–A₂ coordination, coralyne causes a change in the conformation of A₂₀ from that of a random coil to a folded structure, thus enabling the fluorophore to be close to the quencher. As a result, fluorescence quenching occurs between the two organic dyes. When the A₂₀·coralyne probe encounters the diadenosine nucleotide, the resulting complex of coralyne and diadenosine nucleotides forces the removal of coralyne from the probe. Such a conformational change in the probe leads to the restoration of fluorescence. Within a short analysis time of 1 min, the proposed probe provides high selectivity toward c-di-AMP and Ap5A over other common nucleotides. The probe’s detection limit at a signal-to-noise ratio of 3 for both c-di-AMP and Ap5A were estimated to be 0.4 and 4 μM, respectively. The practicality of the proposed probe was demonstrated by quantifying c-di-AMP in bacteria lysates and Ap5A in human tears

Combined Tween 20-Stabilized Gold Nanoparticles and Reduced Graphite Oxide–Fe₃O₄ Nanoparticle Composites for Rapid and Efficient Removal of Mercury Species from a Complex Matrix

This study describes a simple method for removing mercuric ions (Hg²⁺) from a high-salt matrix based on the use of Tween-20-stabilized gold nanoparticles (Tween 20-Au NPs) as Hg²⁺ adsorbents and composites of reduced graphite oxide and Fe₃O₄ NPs as NP collectors. Citrate ions adsorbed on the surface of the Tween 20-Au NPs reduced Hg²⁺ to Hg⁰, resulting in the deposition of Hg⁰ on the surface of the NPs. To circumvent time-consuming centrifugation and transfer steps, the Hg⁰-containing gold NPs were collected using reduced graphite oxide–Fe₃O₄ NP composites. Compared with the reported NP-based methods for removing Hg²⁺, Tween 20-Au NPs offered the rapid (within 30 min), efficient (>99% elimination efficiency), durable (>10 cycles), and selective removal of Hg²⁺, CH₃Hg⁺, and C₂H₅Hg⁺ in a high-salt matrix without the interference of other metal ions. This was attributed to the fact that the dispersed Tween 20-Au NPs exhibited large surface-area-to-volume ratio to bind Hg²⁺ through Hg²⁺–Au⁺ metallophilic interactions in a high-salt matrix. The formation of graphite oxide sheets and reduced graphite oxide–Fe₃O₄ NP composites was demonstrated using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectrometry, and transmission electron microscopy. The mechanism of interaction between Tween 20-Au NPs and Hg²⁺ was studied using visible spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy