Highly Sensitive Adsorption and Detection of Iodide in Aqueous Solution by a Post-Synthesized Zirconium-Organic Framework.

Effective methods of detection and removal of iodide ions (I-) from radioactive wastewater are urgently needed and developing them remains a great challenge. In this work, an Ag+ decorated stable nano-MOF UiO-66-(COOH)2 was developed for the I- to simultaneously capture and sense in aqueous solution. Due to the uncoordinated carboxylate groups on the UiO-66-(COOH)2 framework, Ag+ was successfully incorporated into the MOF and enhanced the intrinsic fluorescence of MOF. After adding iodide ions, Ag+ would be produced, following the formation of AgI. As a result, Ag+@UiO-66-(COOH)2 can be utilized for the removal of I- in aqueous solution, even in the presence of other common ionic ions (NO2-, NO3-, F-, SO42-). The removal capacity as high as 235.5 mg/g was calculated by Langmuir model; moreover, the fluorescence of Ag+@UiO-66-(COOH)2 gradually decreases with the deposition of AgI, which can be quantitatively depicted by a linear equation. The limit of detection toward I- is calculated to be 0.58 ppm.

DNAzyme recognition triggered cascade signal amplification for rapid and highly sensitive visual detection of uranyl ions.

This work presents a rapid and highly sensitive colorimetric assay using bifunctional DNA probe decorated agarose microbeads (MBs) coupled with a cascade signal amplification system, including rolling circle amplification (RCA) and the hemin/G-quadruplex-catalyzed colorimetric reaction, for visualized detection of uranyl ions. The DNA probe integrates the UO22+-specific DNAzyme/substrate as the target recognition unit and a DNA primer as the signal conversion unit. The presence of uranyl ions induces the efficient cleavage of the DNA substrates with the catalysis of DNAzyme. Then the conjugated primers are released from MBs, initiating the RCA reaction (the first amplification). The RCA product consists of repetitive G-quadruplexes that can lead to a second amplification by catalyzing the oxidation of ABTS2- with hemin binding, resulting in a coloration that is visible to the naked eye. The whole assay procedure could be finished within 40 min, including recognition of uranyl and DNA cleavage (5 min), the RCA reaction (30 min) and data readout either by eye or using a UV-vis spectrometer (5 min for each sample). In the optimal conditions, concentrations as low as 5 nM uranyl ions could be distinguished by the naked eye. With UV-vis spectrometric measurement, the visible absorbance had a linear relationship with the concentration of uranyl ions with a dynamic range from 1 nM to 50 nM, and a low detection limit of 0.48 nM (i.e. ∼0.12 ppb) was obtained. Excellent selectivity and anti-interference capability in water samples were also certified. This facile visualized assay could be applied in detecting trace-level uranium for on-site environmental analysis.

Plasmon catalytic PATP coupling reaction on Ag-NPs/graphite studied via in situ electrochemical surface-enhanced Raman spectroscopy.

The p-aminothiophenol (PATP) coupling reaction on plasmon substrates such as Ag and Au nanoparticles has received extensive attention since the catalytic effect of the surface plasmon was found. Currently, in situ kinetic studies of this reaction are rare, especially those focusing on the specific role of the hot electron-hole carriers. Here, in situ electrochemical surface-enhanced Raman spectroscopy (SERS) is developed to study the plasmon catalytic reaction of PATP in a controlled aqueous environment involving the factors of O2, electron and hole carriers, and solution pH. Ag nanoparticles supported on graphite serve as a SERS substrate, which could separate hot electron-hole pairs effectively and is beneficial to study the effects of hot carriers on plasmon-driven reactions. In situ electrochemical SERS measurements reveal two reaction paths for the PATP coupling reaction. One is that plasmon-induced hot holes activate the dehydrogenation of PATP and then the radical coupling reaction to form p,p'-dimercaptoazobenzene (DMAB) under O2-free conditions. Another is likely to be that the surface Ag2O/AgOH, which is generated from Ag and 1O2/O2-, catalyzes the oxidation of PATP and then the coupling process under O2-rich conditions. Benefitting from the potential/atmosphere controlled measurements in situ, the intermediate species of PATP(NH)/PATP(N) are observed with vibrational bands at around 1056, 1202, 1253, 1395, 1514 and 1540 cm-1.

In Situ Surface-Enhanced Raman Spectroscopy Detection of Uranyl Ions with Silver Nanorod-Decorated Tape.

Surface-enhanced Raman spectroscopy (SERS) has been utilized for rapid analysis of uranyl ions (UO2 2+) on account of its fast response and high sensitivity. However, the difficulty of fabricating a suitable SERS substrate for in situ analysis of uranyl ions severely restricts its practical application. Hence, we proposed flexible and adhesive SERS tape decorated with silver nanorod (AgNR) arrays for in situ detection of UO2 2+. The SERS tape was fabricated through a simple "paste & peel off" procedure by transferring the slanted AgNR arrays from silicon to the transparent tape surface. UO2 2+ can be easily in situ detected by placing the AgNR SERS tape into an aqueous solution or pasting it onto the solid matrix surface due to the excellent transparent feature of the tape. The proposed SERS tape with well-distributed AgNRs effectively improved the reproducibility and sensitivity for UO2 2+ analysis. UO2 2+ with concentration as low as 100 nM was easily detected. Besides, UO2 2+ adsorbed on an iron disc and rock surface also can be rapidly in situ detected. With its simplicity and convenience, the AgNR SERS tape-based SERS technique offers a promising approach for environmental monitoring and nuclear accident emergency detection.

Flexible and adhesive tape decorated with silver nanorods for in-situ analysis of pesticides residues and colorants.

A flexible adhesive tape decorated with SERS-active silver nanorods (AgNRs) in the form of an array nanostructure is described. The tape was constructed by transferring the AgNRs nanostructures from silicon to the transparent tape by a "paste & peel off" procedure. The transparent, sticky, and flexible properties of commercial tapes allow almost any SERS-inactive irregular surface to be detected in-situ by pasting the SERS tape onto the position to be analyzed. Three examples for an analytical application are presented, viz. determination of (a) tetramethylthiuram disulfide and thiabendazole (two pesticides), (b) colorants in the gel of a writing pen, and (c) the fluorophore Rhodamine B. The tetramethylthiuram disulfide on apple surface was rapidly detected with a LOD of 28.8 ng·cm-2. The AgNRs effectively quenched the fluorescence of the matrix and fluorophores, this enabling the colorants and Rhodamine B to be identified. The results demonstrated that the SERS tape can be used for versatile in-situ detection. Conceivably, it may find applications in food analysis, non-invasive identification, environmental monitoring, and in other areas of daily life. Graphic abstract A flexible and adhesive SERS active tape decorated with silver nanorods (AgNRs) arrays was constructed through a "paste & peel off" method. It can be used as a versatile in situ analysis platform for various applications.

The contribution of photoinduced charge-transfer enhancement to the SERS of uranyl(VI) in a uranyl-Ag2O complex.

Charge-transfer (CT) is an important enhancement mechanism in the field of surface-enhanced Raman scattering (SERS) that typically increases the Raman intensity of molecules by as much as 10-100 times. Herein, a low-cost Ag2O aggregates substrate was prepared via a facile chemical precipitation method, and the calculated CT-based enhancement factor of the uranyl ions adsorbed on it reached as high as 105, a metal-comparable value. The efficient photoinduced CT process from the valence band of Ag2O to the LUMO of uranyl ions under appropriate excitation sources resulted in the repulsion of the axial oxygen atoms of the OUO bond, which enhanced its polarizability, creating a more intense Raman mode. To the best of our knowledge, this study firstly reports such a strong photoinduced CT enhancement of uranyl ions, with concentrations of 10-8 mol L-1 or lower being detected using this Ag2O substrate. Most importantly, this research has shown that the photoinduced CT enhancement also contributes to the SERS of uranyl ions on pure Ag substrates which have often been ascribed to the electromagnetic enhancement in previous studies. In addition, Ag2O can be used to selectively detect uranyl ions without interference from many other molecules or ions because of the energy matching rule of the photoinduced CT process, which was readily available for uranyl detection in the environmental aqueous solution.

Rapid and sensitive detection of uranyl ion with citrate-stabilized silver nanoparticles by the surface-enhanced Raman scattering technique.

Uranium contamination poses a huge threat to human health due to its widespread use in the nuclear industry and weapons. We proposed a simple and convenient wet-state SERS method for uranyl detection based on the citrate-stabilized silver nanoparticles. The effect of citrate on the detection performance was also discussed. By using the citrate as an internal reference to normalize the peak of uranyl, a quantitative analysis was achieved and a good linear relationship of uranyl concentration from 0.2 to 5 µM with the limit of detection of 60 nM was obtained. With its simplicity, convenience and cost-effectiveness, this method has great potential for the detection of other molecules also.

Surface-Enhanced Raman Scattering Detection of Pesticide Residues Using Transparent Adhesive Tapes and Coated Silver Nanorods.

The efficient extraction of analytes from complex and severe environments is significant for promoting the surface-enhanced Raman scattering (SERS) technique to actual applications. In this paper, a proof-of-concept strategy is proposed for the rapid detection of pesticide residues by utilizing the flexible, transparent, and adhesive properties of commercial tapes and SERS performance of Al2O3-coated silver nanorod (AgNR@Al2O3) arrays. The function of tapes is to rapidly transfer the analytes from the actual surface to the SERS substrate. The novel "tape-wrapped SERS (T-SERS)" approach was constructed by a simple "paste, peel off, and paste again" procedure. The easily obtained but clearly distinguished SERS signals allow us to quickly determine the constituents of complex surfaces, such as tetramethylthiuram disulfide and thiabendazole pesticides from fruits and vegetables, which may be practically applied to food safety, environmental monitoring, and industrial production process controlling.

Simultaneous fluorescent detection of multiple metal ions based on the DNAzymes and graphene oxide.

A novel fluorescent detection strategy for simultaneous detection of Cu2+, Pb2+ and Mg2+ based on DNAzyme branched junction structure with three kinds of DNAzymes and graphene oxide (GO) was presented. Three fluorophores labeled DNA sequences consisted with enzyme-strand (E-DNA) and substrate strand (S-DNA) were annealed to form DNAzyme branched junction structure. In the presence of target metal ion, the DNAzyme was activated to cleave the fluorophore labeled S-DNA. The S-DNA fragments were released and adsorbed onto GO surface to quench the fluorescent signal. The detection limit was calculated to be 1 nM for Cu2+, 200 nM for Mg2+, and 0.3 nM for Pb2+, respectively. This strategy was successfully used for simultaneous detection of Cu2+, Mg2+ and Pb2+ in human serum. Moreover, it had potential application for simultaneous detection of multiple metal ions in environmental and biological samples.

Self-assembly of silver nanoparticles as high active surface-enhanced Raman scattering substrate for rapid and trace analysis of uranyl(VI) ions.

A facile surface-enhanced Raman scattering (SERS) substrate based on the self-assembly of silver nanoparticles on the modified silicon wafer was obtained, and for the first time, an advanced SERS analysis method basing on this as-prepared substrate was established for high sensitive and rapid detection of uranyl ions. Due to the weakened bond strength of OUO resulting from two kinds of adsorption of uranyl species ("strong" and "weak" adsorption) on the substrate, the ν1 symmetric stretch vibration frequency of OUO shifted from 871cm-1 (normal Raman) to 720cm-1 and 826cm-1 (SERS) along with significant Raman enhancement. Effects of the hydrolysis of uranyl ions on SERS were also investigated, and the SERS band at ~826cm-1 was first used to approximately define the constitution of uranyl species at trace quantity level. Besides, the SERS intensity was proportional to the variable concentrations of uranyl nitrate ranging from 10-7 to 10-3molL-1 with an excellent linear relation (R2=0.998), and the detection limit was ~10-7molL-1. Furthermore, the related SERS approach involves low-cost substrate fabrication, rapid and trace analysis simultaneously, and shows great potential applications for the field assays of uranyl ions in the nuclear fuel cycle and environmental monitoring.

Enzyme-free and label-free ultra-sensitive colorimetric detection of Pb(2+) using molecular beacon and DNAzyme based amplification strategy.

An enzyme-free and label-free colorimetric Pb(2+) sensor based on DNAzyme and molecular beacon (MB) has been developed and demonstrated by recycle using enzyme strand for signal amplification. The substrate strand DNA (S-DNA) of DNAzyme could be converted into MB structure with base pairs of stem part at the both ends. The MB could hybridize with enzyme strand DNA (E-DNA) to form DNAzyme, and be activated and cleaved in the presence of Pb(2+). The cleaved MB is much less stable, releasing from the DNAzyme as two product pieces. The product pieces of MB are flexible and could bind to unmodified AuNPs to effectively stabilize them against salt-induced aggregation. Then, the E-DNA is liberated to catalyze the next reaction and amplify the response signal. By taking advantage of repeated using of E-DNA, our proposed method exhibited high sensitive for Pb(2+) detection in a linear range from 0.05 to 5 nM with detection limit of 20 pM by UV-vis spectrometer. Moreover, this method was also used for determination of Pb(2+) in river water samples with satisfying results. Importantly, this strategy could reach high sensitivity without any modification and complex enzymatic or hairpins based amplification procedures.

Ultrasensitive visual detection of DNA with tunable dynamic range by using unmodified gold nanoparticles and target catalyzed hairpin assembly amplification.

A simple and novel strategy for enzyme-free ultrasensitive DNA detection platform has been present here based on gold nanoparticles (AuNPs) colorimetry and target catalyzed hairpin assembly amplification. Three hairpin auxiliary probes (H1, H2, and H3) are designed with signal-stranded DNA (ssDNA) sticky ends which could effectively stabilize AuNPs against salt-induced aggregation. However, a cascade of assembly steps with H1, H2, and H3 are activated in the presence of the target DNA, followed by a disassembly step in which H3 displaces the target DNA from the complex, freeing the target DNA to catalyze the self-assembly of additional branched junctions. The formed branched junction consisted with dsDNA is stiffer, and cannot prevent salt-induced AuNPs aggregation, corresponding to a red-to-blue color change. The result can be read out by naked eyes or UV-vis spectrometer. The detection limit of this method is 0.1 pM by naked eyes, and this result is comparable or even better than enzyme or hybridization chain reaction (HCR) based amplification AuNPs colorimetric assays. Moreover, the dynamic range of sensor could be tuned by using different concentration of hairpins. Importantly, this strategy provides a versatile ultrasensitive detection platform for the DNA and related filed targets including metal ions, small molecules, proteins, cells et al. by combining with specific DNAzymes and aptamers.

[Design and Fabrication of Silver Nanoparticles/Graphen Complex Substrate and Its Application for Detecting Uranium (â ¥)].

Uranium is one of the important nuclear materials to nuclear industry. Because of the direct disposal of spent fuel, there is still a huge possibility that uranium migrates into the groundwater, causing water contamination. It is of great importance to understand the concentration and their species distribution in aqueous solutions. Surface-Enhanced Raman Scattering (SERS) technique has been widely used for the detection of uranium (Ⅵ). However, the interactions between uranium (Ⅵ) and SERS substrate cause the symmetric stretching vibration peak of uranium (Ⅵ) shift to low wave number direction, which is unfavorable for confirming the species of uranium (Ⅵ) in aqueous solution. For instance, the normal Raman bands of uranyl in nitric acid solution are 871 cm-1, which belongs to the symmetric stretching mode of UO2+2. However, it moves to 710 cm-1 on the surface of silver nanorods SERS substrtate. What's more, different SERS substrate causes different number of shift. Graphene has advantages of inertness and integrity as well as 2-dimensional thickness. In this paper, graphene-isolated SERS substrate which is silver nanoparticles (AgNPs)/graphene complex substrate, was designed to prevent the interaction between SERS substrate and it was analyzed by using the inert graphene layer. First of all, according to our previous work, AgNPs SERS substrate was fabricated on silicon wafer by using an ascorbic acid-actived self-assembly method. Then, AgNPs/graphene complex substrate was prepared by transfering monolayer graphene onto the self-assembly AgNPs substrate. The morphology of complex substrate was obtained by SEM. Some AgNPs link together closely to form nanochain structures. Nanochain structures were distributed evenly on the surface of silicon wafer. The 2-dimensional thickness of graphene did not affect the morphology of AgNPs. When using the complex substrate to detect uranyl nitrate (5×10-4 mol·L-1)，the Raman peak that appeared around 771 cm-1 is considered to be the symmetric stretching mode of UO2+2, shifting back about 52 cm-1 to high wave number direction when compared with AgNPs substrate, which was about ~719 cm-1. The result indicates that graphene layer isolates the interaction between AgNPs substrate and uranyl in some degree.