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Spectroscopic characterization of clusters is crucial to understanding the structures and reaction mechanisms at the microscopic level, but it has been proven to be a grand challenge for neutral clusters because the absence of a charge makes it difficult for the size selection and detection. Infrared (IR) spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser (VUV-FEL) has recently been developed in the lab. The IR-VUV depletion and IR+VUV enhancement spectroscopic techniques open new avenues for size-selected IR spectroscopies of a large variety of neutral clusters without confinement (i.e., an ultraviolet chromophore, a messenger tag, or a host matrix). The spectroscopic principles have been demonstrated by investigations of some neutral water clusters and some metal carbonyls. Here, the spectroscopic principles and their applications for neutral clusters are reviewed.

The series of heterodinuclear metal oxide carbonyls in the form of TaNiO(CO)n− (n = 5–8) are generated in the pulsed-laser vaporization source and characterized by mass-selected photoelectron velocity-map spectroscopy. During the consecutive CO adsorption, the μ2-O-bent structure initially is the most favorable for TaNiO(CO)5−, and subsequently both μ2-O-bent and μ2-O-linear structures are degenerate for TaNiO(CO)6−, then the μ2-O-linear structure is most preferential for TaNiO(CO)7−, and finally the η2-CO2-tagged structure is the most energetically competitive one for TaNiO(CO)8−, i.e., the CO oxidation occurs at n = 8. In contrast to the literature reported CO oxidation on heteronuclear metal oxide complexes generally proceeding via Langmuir–Hinshelwood-like mechanism, complementary theoretical calculations suggest that both Langmuir–Hinshelwood-like and Eley–Rideal-like mechanisms prevail for the CO oxidation reaction on TaNiO(CO)8− complex. Our findings provide new insight into the composition-selective mechanism of CO oxidation on heteronuclear metal complexes, of which the composition be tailored to fulfill the desired chemical behaviors.

Neutral titanium-metal carbonyl complexes with the chemical formula Ti(CO)n (n = 4-7) are produced in the gas phase by the reactions of titanium atoms with carbon monoxide in a pulsed laser vaporization-supersonic expansion source. Their infrared absorption spectra in the carbonyl stretching frequency region are measured by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free electron laser. Infrared spectroscopy in conjunction with quantum chemical calculations confirm that all of these complexes have unexpected titanium ketenylidene OTiCCO(CO)n-2 structures. Bonding analysis indicates that the OTiCCO core structure can be described by the bonding interactions between a TiO+ cation in the doublet ground state and a doublet ground state of CCO-. The results reveal that the C-O bond breaking and C-C bond formation proceed efficiently in the reactions between laser-vaporized titanium atoms and carbon monoxide.

The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, [(CH $_3$ NH $_2$ )(H $_2$ O) $_n$ ] $^+$ ( $n$ =1 $-$ 5), have been studied using quantum chemical calculations at the MP2/6-311+G(2d,p) level. The results reveal that the formation of proton-transferred CH $_2$ NH $_3$ $^+$ ion core structure is preferred via the intramolecular proton transfer from the methyl group to the nitrogen atom and the water molecules act as the acceptor for the O $\cdots$ HN hydrogen bonds with the positively charged NH $_3$ $^+$ moiety of CH $_2$ NH $_3$ $^+$ , whose motif is retained in the larger clusters. The CH $_3$ NH $_2$ $^+$ ion core structure is predicted to be less energetically favorable. Vibrational frequencies of CH stretches, hydrogen-bonded and free NH stretches, and hydrogen-bonded OH stretches in the calculated IR spectra of the CH $_2$ NH $_3$ $^+$ and CH $_3$ NH $_2$ $^+$ type structures are different from each other, which would afford the sensitive probes for fundamental understanding of hydrogen bonding networks generated from the radiation-induced chemical processes in the [(CH $_3$ NH $_2$ )(H $_2$ O) $_n$ ] $^+$ complexes.

A special electrochemical sensor was developed based on molecularly imprinted polymer (MIP), which was formed by the dopamine (DA) self-polymerization on the surface of Pd-Cu bimetallic alloy functionalized poly (diallyldimethylammonium chloride) -dispersed graphene (PDDA-Gr). This sensor takes advantages of molecularly imprinted technique and nanocomposite to realize the application of amaranth determination with high selectivity and sensitivity. In this study, the nanocomposite PDDA-Gr-(Pd-Cu) characterized by various techniques provides large surface area and accelerates the electron transfer process. The imprinted composite deposited on the surface of PDDA-Gr-(Pd-Cu), not only provides abundant specific cavities to amaranth, but also decreases the thickness of molecularly imprinted polymer leading to high conductivity. The electrochemical behavior of amaranth on the modified electrode was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). After optimization, the anodic peak current showed a linear relationship with the concentration of amaranth in the range of 0.006–10 μM, and the limit of detection (LOD) was 2 nM. The proposed sensor distinguished amaranth from other structurally similar substances such as sunset yellow and tartrazine with good results. In addition, the molecularly imprinted sensor proposed in this study was successfully applied to determine amaranth in the soft drink with satisfactory recoveries.

The mass-selected infrared photodissociation (IRPD) spectroscopy was utilized to investigate the interactions of cationic cobalt with carbon dioxide molecules. Quantum chemical calculations were performed on the [Co(CO $_2$ ) $_n$ ]+clusters to identify the structures of the low-lying isomers and to assign the observed spectral features. All the [Co(CO $_2$ ) $_n$ ]+ ( $n$ =2 $-$ 6) clusters studied here show resonances near the CO $_2$ asymmetric stretch of free CO $_2$ molecule. Experimental and calculated results indicate that the CO $_2$ molecules are weakly bound to the Co+ cations in an end-on configuration via a charge-quadrupole electrostatic interaction. The present IRPD spectra of [Co(CO $_2$ ) $_n$ ]+ clusters have been compared to those of Ar-tagged species ([Co(CO $_2$ ) $_n$ ]+-Ar), which would provide insights into the tagging effect of rare gas on the weakly-bounded clusters.

Three novel electrochemical sensors constructed by Pd-Pt bimetallic nanocages, Pt-Cu bimetallic nanoframes and Co-Ni bimetallic nanoflowers and poly (diallyldimethylammonium chloride) (PDDA)-dispersed graphene (Gr) were applied to determine sunset yellow (SY, E110). The PDDA-Gr-(Pd-Pt), PDDA-Gr-(Pt-Cu) and PDDA-Gr-(Co-Ni) nanocomposites formed different nanostructure with unique features and were characterized by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The special morphology of the three bimetallic nanoparticles endows the designed electrochemical sensors with large active areas and good electrical conductivity, which provides more active sites for the target analyte and improves the sensitivity of the determination of sunset yellow. The developed sensors were investigated and compared with each other on the determination of SY by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Under optimal experimental condition, the anodic peak currents of sunset yellow on three modified electrodes was linearly correlated with the concentration of sunset yellow in the ranges of 0.02–10.0, 0.02–10.0, 0.008–10.0 μM, respectively. The limits of detection (LOD) of SY were 6.0, 4.0 and 2.0 nM, respectively. In addition, the three new proposed sensors in this study were successfully applied to determine SY in real soft drink sample. This study provides a new platform for designing sensitive electrochemical sensor based on non-noble metals with low cost.

An electrochemical sensing platform was fabricated based on composition of poly(sodium 4-styrenesulfonate) (PSS), graphene (GN) and WO3 nanorods on glassy carbon electrode (GCE). The PSS-GN/WO3 nanocomposite significantly increased the oxidative activity of puerarin due to the individual merit and mutual effect of PSS-GN and WO3 nanorods which improved the performance of the electrochemical sensor with high sensitivity. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were used for the detection of puerarin. Under the optimal conditions, the oxidation peak currents of puerarin increased proportionally to the concentration within the range of 0.06-0.6μmolL-1 and 0.6-6.0μmolL-1, and the limit of detection was 0.04μmolL-1. The proposed modified electrode was highly sensitive and was successfully applied to determine puerarin in the pharmaceutical sample and human plasma with satisfactory recoveries. The as-prepared PSS-GN/WO3/GCE electrochemical sensor offers a feasible way for designing simpler, low-cost, no toxic and environment-friendly sensors based on WO3 nanorods.

A new sensitive electrochemical sensor based on poly(sodium 4-styrenesulfonate) (PSS) functionalized graphene and Co3O4 nanoparticle clusters (PSS-GN/Co3O4) ternary composite was fabricated via two-step synthesis. The PSS-GN/Co3O4 nanocomposite significantly increased the oxidative activity of amaranth due to the individual merit and mutual effect of PSS-GN and Co3O4 nanoparticle clusters which improved the performance of the electrochemical sensor with high sensitivity. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were used for the detection of amaranth. Under the optimal conditions, the oxidation peak currents of amaranth increased proportionally to the concentration within the range of 0.01–1.0 and 1.0–6.0 μmol L−1, and the limit of detection was 4.0 nmol L−1. The proposed modified electrode was highly sensitive and was successfully applied to determine amaranth in the soft drinks with satisfactory recoveries. The as-prepared PSS-GN/Co3O4/GCE electrochemical sensor offers a feasible way for designing simpler, low cost, and environment-friendly sensors based on Co3O4 nanoparticle clusters.

A sensitive electrochemical sensor based on sodium dodecyl sulfonate (SDS) functionalized graphene hybrid SnO 2 nanoparticles (SDS-GN/SnO 2 ) composite was fabricated via one-pot synthesis. The SDS-GN/SnO 2 nanocomposite significantly increased the redox activity of daphnetin due to the individual merit and mutual effect of SDS-GN and SnO 2 nanoparticles which improved the performance of the electrochemical sensor with high sensitivity. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were used for the detection of daphnetin. Under the optimal conditions, the oxidation peak currents of daphnetin increased proportionally to the concentration within the range of 0.03–1.0 μmol L − 1 and 1.0–8.0 μmol L − 1 , and the limit of detection was 8.0 nmol L − 1 . The proposed modified electrode was highly sensitive and was successfully applied to determine daphnetin in the pharmaceutical sample with satisfactory recoveries. The as-prepared SDS-GN/SnO 2 /GCE electrochemical sensor offers a feasible way for designing simpler, low cost sensors based on SnO 2 nanoparticles.

A new and sensitive electrochemical sensor for simultaneous determination of sunset yellow and tartrazine was constructed by poly (diallyldimethylammonium chloride) (PDDA)-dispersed graphene (Gr) and palladium nanoparticle (Pd NP) composite. The PDDA-Gr-Pd composite displayed good selectivity and high sensitivity to the targets, which were ascribed to the synergistic effect of the large surface area and good electron transfer efficiency of both Gr and Pd NPs. The electrochemical properties of sunset yellow and tartrazine on the modified electrode were investigated by cyclic voltammetry and differential pulse voltammetry. Under the optimum experimental conditions, the peak currents were linear with the concentrations of sunset yellow and tartrazine in the ranges of 0.01–10.0 and 0.05–8.0 μmol L−1, respectively. The limit of detection (LOD) was 2.0 and 5.0 nmol L−1 for sunset yellow and tartrazine, respectively. The developed simple and sensitive method was successfully applied for simultaneous determination of sunset yellow and tartrazine in soft drinks with satisfactory results.