Surface-Enhanced Raman Scattering of Phenols and Catechols by a Molecular Analogue of Titanium Dioxide

Molecular-Sieving Label-Free Surface-Enhanced Raman Spectroscopy for Sensitive Detection of Trace Small-Molecule Biomarkers in Clinical Samples

Small-molecule biomarkers are ubiquitous in biological fluids with pathological implications, but major challenges persist in their quantitative analysis directly in complex clinical samples. Herein, a molecular-sieving label-free surface-enhanced Raman spectroscopy (SERS) biosensor is reported for selective quantitative analysis of trace small-molecule trimetazidine (TMZ) in clinical samples. Our biosensor is fabricated by decorating a superhydrophobic monolayer of microporous metal-organic frameworks (MOF) shell-coated Au nanostar nanoparticles on a silicon substrate. The design strategy principally combines the hydrophobic surface-enabled physical confinement and preconcentration, MOF-assisted molecular enrichment and sieving of small molecules, and sensitive SERS detection. Our biosensor utilizes such a "molecular confinement-and-sieving" strategy to achieve a five orders-of-magnitude dynamic detection range and a limit of detection of ≈0.5 nM for TMZ detection in either urine or whole blood. We further demonstrate the applicability of our biosensing platform for longitudinal label-free SERS detection of the TMZ level directly in clinical samples in a mouse model.

Amino-Functionalized NDI-Based MOFs as Unusual "Turn On" and "Turn Off" Fluorescent Sensors for Phenolic Pollutants with Double Solvent Channel Response and Iodine Adsorbents

Regulating mixed ligands to change the functional properties of metal-organic frameworks (MOFs) has been an important topic; especially, the structural changes have significant implications for the transformation of sensing response in different solvent channels. Herein, two [Cd (DPNDI) (NH2-BDC)0.5(NO3)]·2.25DMF (1) and [Cd(DPNDI)(NH2-AIPA)]·0.5DMF (2) (DPNDI = N,N-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide, NH2-BDC = 2-amino terephthalic acid, NH2-AIPA = 5-aminoisophthalic acid) were synthesized by the solvothermal method. Structural analysis shows that complex 1 has a two-dimensional planar network structure and complex 2 exhibits a three-dimensional network structure, endowing its potential as an efficient fluorescence sensor for phenolic compound detection under different solvent environments. Both complexes showed high fluorescence quenching sensitivity to phenolics in a water medium. Conversely, complex 1 showed a fluorescence enhancement response to phenolic pollutants in an ethanol system with significantly low detection limits and recyclability. The detection limits were 0.58 μM for TNP, 1.3 μM for DNP, and 2.43 μM for PCP. In addition, the uncoordinated amino groups in the complexes promote them to exhibit excellent iodine adsorption performance. Especially, complex 2 can serve as an adsorbent for iodine in cyclohexane solution with better adsorption efficiency than that of complex 1, and its adsorption capacity can reach 505 mg/g. The mixed ligands regulation strategy of NDI-based MOFs will open up an effective avenue for the conversion of fluorescence signals in dual-solvent channels and play simultaneously important roles in multiple applications.

Electrically weldable conductive elastomers

Flexible and stretchable electronic devices are subject to failure because of vulnerable circuit interconnections. We develop a low-voltage (1.5 to 4.5 V) and rapid (as low as 5 s) electric welding strategy to integrate both rigid electronic components and soft sensors in flexible circuits under ambient conditions. This is achieved through the design of conductive elastomers composed of borate ester polymers and conductive fillers, which can be self-welded and generate welding effects to various materials including metals, hydrogels, and other conductive elastomers. The welding effect is generated through the electrochemical reaction-triggered exposure of interfacial adhesive promotors or the cleavage/reformation of dynamic bonds. Our strategy can ensure both mechanical compliance and conductivity at the circuit interfaces and easily produce welding strengths in the kilopascal to megapascal range. The as-designed conductive elastomers in combination with the electric welding technique provide a robust platform for constructing standalone flexible and stretchable electronic devices that are detachable and assemblable on demand.

Structural Chemistry of Titanium (IV) Oxo Clusters, Part 2: Clusters without Carboxylate or Phosphonate Ligands

Homometallic titanium oxo clusters (TOC) are one of the most important groups of metal oxo clusters. In a previous article, TOC structures with carboxylato and phosphonato ligands were reviewed and categorized. This work is now extended to clusters with other ligands. Comparison of the different cluster types shows how the interplay between condensation of the titanium polyhedra by means of bridging oxygen atoms and the coordination characteristics of the ligands influences the cluster structures and allows working out basic construction principles of the cluster core.

A cryptand-like Ti-coordination compound with visible-light photocatalytic activity in CO2 storage

A cryptand-like Ti-coordination compound, namely Ti12Cs, comprising two Ti6-salicylate cages and hosting two Cs+ ions, was synthesized by the solvothermal method. It exhibits strong visible-light absorption with an absorption band edge of 652 nm, attributed to the electron transition from salicylate ligands to Ti ions. Electrochemical impedance, visible-light transient photocurrent response, and photoluminescence spectra confirm that Ti12Cs has excellent visible-light response and charge-separation properties. Ti12Cs can be used as a heterogeneous and recyclable photocatalyst for CO2/epoxide cycloaddition, with high utilization efficiency of visible-light under mild conditions. The mechanism investigation points to a synergistic effect of photocatalysis and Lewis acid catalysis.

Ligation of Titanium-oxide and {Mo2} Units for Solar CO2 Storage

Two Mo-Ti-mixed oxide clusters, Ti6Mo4 and Ti4Mo4, which contain the {Mo2V} unit commonly observed in many polyoxomolybdates, were successfully synthesized. The introduction of a {Mo2V} dopant into a titanium-oxide cluster (TOC) results in a red shift of the absorption edge, hence leading to a substantial enhancement of visible-light absorption. The band gap electron transition mainly involves the ligand-to-metal charge transfer (LMCT, benzoate-to-Mo) and MoV d-d transition. Both clusters show favorable visible-light responsiveness and charge-separation efficiency. Both serve as heterogeneous photocatalysts and exhibit excellent catalytic activity in CO2/epoxide cycloadditions under very mild conditions. The mechanism study suggests that the catalytically active sites are mainly MoV, and the photogenerated electrons and holes are both involved. Ti6Mo4 exhibits better photocatalytic activity than Ti4Mo4, demonstrating the crucial role of the titanium-oxide core, which corresponds to improved light absorption and charge-separation efficiency. Our findings highlight the potential of the {Mo2V} unit in constructing Mo-Ti-mixed oxide clusters with interesting topologies and excellent solar-light-harvesting activity.

Quantitative detection of purine from food products with different water activities using needle-based surface-enhanced Raman scattering sensors

Typically, for accurate quantitative tests of molecules, considering the actual solute concentration in the environment with different water activities (Aws) is essential. Accordingly, for effective detection of food substances, this paper proposes a non-destructive pluggable sensor to capture and monitor four free purines based on surface-enhanced Raman scattering characteristics such as sensitivity, uniformity, repeatability, and stability. In particular, we investigate the impact of Aw on the evaluation of purine detection and its deviation corrections. Furthermore, the recoveries of purine from three food products, including fish (Aw: 0.99), ham (Aw: 0.91), and bacon (Aw: 0.73), are subsequently explored to validate the reliability of the proposed method. The results indicate that the proposed non-destructive pluggable sensor performs better when the Aw is considered. Therefore, this strategy for achieving more reliable quantitative detection by rectifying deviations based on the Aw can significantly help monitor food quality.

Quantitative Label-Free SERS Detection of Trace Fentanyl in Biofluids with a Freestanding Hydrophobic Plasmonic Paper Biosensor

The prevalence of fentanyl abuse raises global public health concerns with an unprecedented surge in overdose deaths. Rapid identification and quantification of fentanyl in biofluids is of paramount importance to combat fentanyl abuse for law enforcement agencies and promptly treat patients for medical professionals. Herein, a freestanding surface-enhanced Raman spectroscopy (SERS) biosensor with excellent condensing enrichment capability, termed FrEnSERS biosensor, is reported for quantitative label-free detection of trace fentanyl in biofluids. This biosensor comprises a reduced graphene oxide membrane decorated with high-density hydrophobic Au nanostars. A combination of the high SERS enhancement and the focusing effect for analyte enrichment of the hydrophobic surface accounts for the remarkable SERS performance of the FrEnSERS biosensor. We demonstrate that the FrEnSERS biosensor achieves the sensitive and quantitative detection of fentanyl in both serum and urine over a wide dynamic range spanning more than 4 orders of magnitude, with a limit of detection of 0.47 ng/mL for serum samples and 0.73 ng/mL for urine samples. Our biosensor is sensitive, cost-effective, and reliable for rapid quantitative analysis of fentanyl in biofluids with great promise for forensic analysis and clinical diagnosis.

Nanoscale Surface-Enhanced Raman Spectroscopy Investigation of a Polyphenol-Based Plasmonic Nanovector

The demand for next-generation multifunctional nanovectors, combining therapeutic effects with specific cellular targeting, has significantly grown during the last few years, pursuing less invasive therapy strategies. Polyphenol-conjugated silver nanoparticles (AgNPs) appear as potential multifunctional nanovectors, integrating the biorecognition capability and the antioxidant power of polyphenols, the antimicrobial activity of silver, and the drug delivery capability of NPs. We present a spectroscopic and microscopic investigation on polyphenol-synthesized AgNPs, selecting caffeic acid (CA) and catechol (CT) as model polyphenols and using them as reducing agents for the AgNP green synthesis, both in the presence and in the absence of a capping agent. We exploit the plasmonic properties of AgNPs to collect Surface-Enhanced Raman Scattering (SERS) spectra from the nanosized region next to the Ag surface and to characterize the molecular environment in the proximity of the NP, assessing the orientation and tunable deprotonation level of CA, depending on the synthesis conditions. Our results suggest that the SERS investigation of such nanovectors can provide crucial information for their perspective biomedical application.

New picolinate-functionalized titanium-oxide clusters: syntheses, structures and photocatalytic H2 evolution

Two nanosized titanium-oxide clusters (TOCs), Ti₁₂(μ₂-O)₁₄(μ₃-O)₄PA₁₆ (1; PA = 2-picolinate) and Ti₁₂(μ₂-O)₁₈PA₁₈ (2) were synthesized by using 2-picolinic acid and Ti(OⁱPr)₄ in one-pot reactions. Their structures were determined using single-crystal X-ray diffractometry. Although both have the same core composition of Ti₁₂O₁₈, 1 exhibited superior H₂ evolution activity of up to 180 μmol h^-1 g^-1, which is nearly eight times faster than 2. Mechanism studies revealed that 1 could induce the assembly of 2.3 nm PtNPs into 10-30 nm supra-nanoparticle structures, which contributed to the increased H₂ evolution rate.

Simple, rapid, and visual electrochemiluminescence sensor for on-site catechol analysis

Environmental pollution caused by aromatic compounds such as catechol (Cat) has become a major issue for human health. However, there is no simple, rapid, and low-cost method for on-site monitoring of Cat. Here, based on ECL quenching mechanism, we develop a simple, rapid and visual mesoporous silica (MSNs)-electrochemiluminescence (ECL) sensor for on-site monitoring of Cat. The mechanism of ECL quenching is due to the interaction between Cat and Ru(bpy)₃^2+* and the interactions between the oxidation products of Cat and DBAE. MSNs films with ordered perpendicular mesopore channels exhibit an amplification effect of ECL intensity due to the negatively charged pore channel. There is a good linear relationship between ECL intensity and Cat concentration in the range of 10 ∼ 1000 μM with the limit of detection (LOD) of 9.518 μM (R² = 0.99). The on-site sensor is promising to offer new opportunities for pharmaceuticals analysis, on-site monitoring, and exposure risk assessment.

A simple, rapid and visual mesoporous silica (MSNs)-electrochemiluminescence (ECL) sensor was developed for on-site monitoring of Cat.

"Three-in-One" Structural-Building-Mode-Based Ti16-Type Titanium Oxo Cluster Entirely Protected by the Ligands Benzoate and Salicylhydroxamate

Titanium oxo clusters (TOCs) with accurate molecular structures have potential applications in photocatalysis, such as photocatalytic degradation, hydrogen production, and water oxidation. The hydrolytic stability and light absorption ability of TOCs have important impacts on photocatalysis, where the selection of peripheral organic ligands plays a significant role. In this regard, salicylhydroxamic acid (abbreviated as H₃L) attracts our attention, acting as a ligand for its multidentate and dye-functional features, which can increase the hydrolytic stability and broaden light absorption for TOCs. Herein, two TOCs were solvothermally synthesized and structurally characterized using H₃L, formulated as [Ti₈(μ₂-O)₂(μ₃-O)₂(OⁱPr)₁₂(L)₄]·2CH₃CN (1) and [Ti₁₆(μ₂-O)₁₀(μ₃-O)₄(PhCOO)₁₄(L)₆(HL)₂]·4CH₃CN·2ⁱPrOH (2). Complex 2 was obtained by adding excessive benzoic acid over the reaction system of 1, resulting in enhanced hydrolytic stability via the replacement of all alkoxy ligands by multidentate ligands for protection. Interestingly, for the first time, the "three-in-one" structural building mode with {Ti₆} + {Ti₄} + {Ti₆} by the common subunits in 2 was observed among all reported TOCs. Moreover, complex 2 can strongly absorb visible light reaching up to 700 nm and exhibit obvious activity for the photodegradation of methyl orange.

Syntheses, Structures and Ligand Binding Modes of Titanium-Oxide Complexes of 2-Picolinate

Six titanium-oxide clusters (TOCs) comprised of 6-19 Ti atoms all of which are of nanometer size were synthesized with the functionalization of 2-picolinate (PA). Their structures were determined by single-crystal...

A Mesoporous Lead-Doped Titanium Oxide Compound with High Performance and Recyclability in I2 Uptake and Photocatalysis

A three-dimensional (3D) mesoporous material with an atomically precise structure, Ti₁₆Pb₅O₁₆(C₆H₅CO₂)₂(OCH₃)₄₀ (Ti₁₆Pb₅), comprised of a novel high-nuclearity Pb-doped titanium oxide cluster (TOC), was synthesized. Ti₁₆Pb₅ exhibited a surface area of 45 m² g^-1 and a pore diameter of 3.5 nm. It exhibited an uptake capacity of I₂ of ≤2.2 g g^-1 in vapor, and the performance was maintained after seven uptake-release cycles. Ti₁₆Pb₅ also showed a high adsorption ratio and capacity (93% and 3.1 g g^-1) in hexane. The characterization data, including Fourier transform infrared, Raman, and powder X-ray diffraction, suggested the lattice structure of Ti₁₆Pb₅ was rigid and I₂ was accommodated in the pores of Ti₁₆Pb₅. To the best of our knowledge, this is the first example of using a TOC in I₂ adsorption. In addition, Ti₁₆Pb₅ showed excellent activity and recyclability in visible-light degradation of dye pollutants and photocurrent generation. Our structural analysis suggested the alkoxide ligands within the channels of Ti₁₆Pb₅ build up a confined polar environment and thereby facilitate I₂ accommodation, and meanwhile, the improved performances and stabilities of Ti₁₆Pb₅ are correlated with its cluster-based, 3D hierarchical structure.

Advanced Optical Sensing of Phenolic Compounds for Environmental Applications

Phenolic compounds are particularly dangerous due to their ability to remain in the environment for a long period of time and their toxic effects. They enter in the environment in different ways, such as waste from paper manufacturing, agriculture (pesticides, insecticides, herbicides), pharmaceuticals, the petrochemical industry, and coal processing. Conventional methods for phenolic compounds detection present some disadvantages, such as cumbersome sample preparation, complex and time-consuming procedures, and need of expensive equipment. Therefore, there is a very large interest in developing sensors and new sensing schemes for fast and easy-to-use methods for detecting and monitoring the phenolic compound concentration in the environment, with special attention to water. Good analytical properties, reliability, and adaptability are required for the developed sensors. The present paper aims at revising the most generally used optical methods for designing and fabricating biosensors and sensors for phenolic compounds. Some selected examples of the most interesting applications of these techniques are also proposed.

Supramolecular Chemistry of Titanium Oxide Clusters

Titanium oxide clusters (TOCs) have been emerging as a new type of inorganic molecular entities of supramolecular chemistry. Herein, a perspective on the structures and functionalities of TOCs over the past three decades is given and the paramount roles of TOCs in serving supramolecular chemistry are demonstrated. Four types of supramolecular assemblies based on TOCs are reviewed, namely, TOC hosts for ion inclusion, mechanically interlocked molecular systems built from cyclic TOCs, reactivities of surface sites toward ligand exchange, and hierarchical structures of TOCs. The principles and advantages of TOCs toward each application are fully discussed, along with structural analyses. Following this path, more functional TOC-based supramolecular systems may be designed and synthesized in the future, which, in turn, will certainly enhance research into both supramolecular and coordination chemistry of titanium.