Pushing the Limits of Capacitively Coupled Contactless Conductivity Detection for Capillary Electrophoresis.

Capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D) has proven to be an efficient technique for the separation and detection of charged inorganic, organic, and biochemical analytes. It offers several advantages, including cost-effectiveness, nanoliter injection volume, short analysis time, good separation efficiency, suitability for miniaturization, and portability. However, the routine determination of common inorganic cations (NH4+, K+, Na+, Ca2+, Mg2+, and Li+) and inorganic anions (F-, Cl-, Br-, NO2-, NO3-, PO43-, and SO42-) in water quality monitoring typically exhibits limits of detection of about 0.3-1 µM without preconcentration. This sensitivity often proves insufficient for the applications of CE-C4D in trace analysis situations. Here, we explore methods to push the detection limits of CE-C4D through a comprehensive consideration of signal and noise sources. In particular, we (i) studied the model of C4D and its guiding roles in C4D and CE-C4D, (ii) optimized the bandwidth and noise performance of the current-to-voltage (I-V) converter, and (iii) reduced the noise level due to the strong background signal of the background electrolyte by adaptive differential detection. We characterized the system with Li+; the 3-fold signal-to-noise (S/N) detection limit for Li+ was determined at 20 nM, with a linear range spanning from 60 nM to 1.6 mM. Moreover, the optimized CE-C4D method was applied to the analysis of common mixed inorganic cations (K+, Na+, Ca2+, Mg2+, and Li+), anions (F-, Cl-, Br-, NO2-, NO3-, PO43-, and SO42-), toxic halides (BrO3-) and heavy metal ions (Pb2+, Cd2+, Cr3+, Co2+, Ni2+, Zn2+, and Cu2+) at trace concentrations of 200 nM. All electropherograms showed good S/N ratios, thus proving its applicability and accuracy. Our results have shown that the developed CE-C4D method is feasible for trace ion analysis in water quality control.

Correction: A compact and high-performance setup of capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D).

Correction for 'A compact and high-performance setup of capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D)' by Lin Li et al., Analyst, 2024, https://doi.org/10.1039/d4an00354c.

A compact and high-performance setup of capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D).

Capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D) has the advantages of high throughput (simultaneous detection of multiple ions), high separation efficiency (higher than 105 theoretical plates) and rapid analysis capability (less than 5 min for common inorganic ions). A compact CE-C4D system is ideal for water quality control and on-site analysis. It is suitable not only for common cations (e.g. Na+, K+, Li+, NH4+, Ca2+, etc.) and anions (e.g. Cl-, SO42-, BrO3-, etc.) but also for some ions (e.g. lanthanide ions, Pb2+, Cd2+, etc.) that require complex derivatization procedures to be detected by ion chromatography (IC). However, an obvious limitation of the CE-C4D method is that its sensitivity (e.g. 0.3-1 µM for common inorganic ions) is often insufficient for trace analysis (e.g. 1 ppb or 20 nM level for common inorganic ions) without preconcentration. For this technology to become a powerful and routine analytical technique, the system should be made compact while maintaining trace analysis sensitivity. In this study, we developed an all-in-one version of the CE-C4D instrument with custom-made modular components to make it a convenient, compact and high-performance system. The system was designed using direct digital synthesis (DDS) technology to generate programmable sinusoidal waveforms with any frequency for excitation, a kilovolt high-voltage power supply for capillary electrophoresis separation, and an "effective" differential C4D cell with a low-noise circuitry for high-sensitivity detection. We characterized the system with different concentrations of Cs+, and even a low concentration of 20 nM was detectable without preconcentration. Moreover, the optimized CE-C4D setup was applied to analyse mixed ions at a trace concentration of 200 nM with excellent signal-to-noise ratios. In typical applications, the limits of detection based on the 3σ criterion (without baseline filtering) were 9, 10, 24, 5, and 12 nM for K+, Cs+, Li+, Ca2+, and Mg2+, respectively, and about 7, 6, 6 and 6 nM for Br-, ClO4-, BrO3- and SO42-, respectively. Finally, the setup was also applied for the analysis of all 14 lanthanide ions and rare-earth minerals, and it showed an improvement in sensitivity by more than 25 times.

Calibration of the oscillation amplitude of quartz tuning fork-based force sensors with astigmatic displacement microscopy.

Quartz tuning forks and qPlus-based force sensors offer an alternative approach to silicon cantilevers for investigating tip-sample interactions in scanning probe microscopy. The high-quality factor (Q) and stiffness of these sensors prevent the tip from jumping to the contact, even at sub-nanometer amplitude. The qPlus configuration enables simultaneous scanning tunneling microscopy and atomic force microscopy, achieving spatial resolution and spectroscopy at the subatomic level. However, to enable precise measurement of tip-sample interaction forces, confidence in these measurements is contingent upon the accurate calibration of the spring constant and oscillation amplitude of the sensor. Here, we have developed a method called astigmatic displacement microscopy with picometer sensitivity.

Flexible optical limiters based on Cu3VSe4 nanocrystals.

Optical limiters are greatly needed to protect eyes and sensitive optoelectronic devices such as photodetectors and sensors from laser damage, but they are currently plagued by low efficiency. In this work, we utilized Cu3VSe4 nanocrystals (NCs) to enhance laser protection performance, and they exhibit higher saturation intensity and broader nonlinear spectral response extending into the near IR region than the C60 benchmark. A flexible optical limiter goggle prototype based on the NCs significantly attenuated the incident laser beam, with Z scan and I scan measurements demonstrating a giant nonlinear absorption coefficient ß value of 1.0 × 10-7 m W-1, a large optical damage threshold of 3.5 J cm-2, and a small starting threshold of 0.22 J cm-2. Transient absorption spectroscopy disclosed that the origin of the excellent nonlinearity was associated with quasi-static dielectric resonance behavior and a large TPA cross-section of 3.3 × 106 GM was measured for Cu3VSe4 NCs, suggesting the potential of intermediate bandgap (IB) semiconductors as alternatives to plasmonic noble metals for ultrafast photonics. Hence, optical limiters based on such semiconductors offer new avenues for laser protection in optoelectronic and defense fields.

After enjoying curry: urine metabolism analysis of curcuminoids by microemulsion electrokinetic chromatography with laser-induced native fluorescence detection.

Considering pH-dependent fluorescence of curcuminoids, a microemulsion electrokinetic chromatographic (MEEKC) method was developed under acidic conditions for their separation and detection using laser-induced native fluorescence (LINF), so as to solve the analysis of urine metabolism for curcuminoids. The microemulsion composition was optimized by response surface methodology (RSM), and the effects of buffer pH and organic modifiers were systematically investigated. The optimal buffer for the separation of curcuminoids was chosen as follows: 2.8% (v/v) ethyl acetate, 80 mM SDS and 2.8% (v/v) n-butanol to form microemulsion, 28% (v/v) ethanol as organic modifier, and 20 mM phosphoric acid as electrolyte at pH 3.0. Under these conditions, four curcuminoids including curcumin, demethoxy curcumin (DMC), bisdemethoxy curcumin (BDMC) and demethyl curcumin (DEC) could be well separated within 18 min, and the detection limits (LOD, based on S/N=3) were calculated to be 71, 60, 22, and 147 pg mL-1, respectively. Combined with solid-phase extraction (SPE), the developed MEEKC-LINF method has been successfully applied to continuously monitor the curcuminoids and related metabolites in human urine collected from a healthy volunteer after oral administration of curry, testifying that this method has potential for evaluating the pharmacological activity of curcuminoids.

Atomic Force Microscopy Based Top-Illumination Electrochemical Tip-Enhanced Raman Spectroscopy.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) is a powerful technique for the in situ study of the physiochemical properties of the electrochemical solid/liquid interface at the nanoscale and molecular level. To further broaden the potential window of EC-TERS while extending its application to opaque samples, here, we develop a top-illumination atomic force microscopy (AFM) based EC-TERStechnique by using a water-immersion objective of a high numerical aperture to introduce the excitation laser and collect the signal. This technique not only extends the application of EC-TERS but also has a high detection sensitivity and experimental efficiency. We coat a SiO2 protection layer over the AFM-TERS tip to improve both the mechanical and chemical stability of the tip in a liquid TERS experiment. We investigate the influence of liquid on the tip-sample distance to obtain the highest TERS enhancement. We further evaluate the reliability of the as-developed EC-AFM-TERS technique by studying the electrochemical redox reaction of polyaniline. The top-illumination EC-AFM-TERS is promising for broadening the application of EC-TERS to more practical systems, including energy storage and (photo)electrocatalysis.

Electrochemical Tip-Enhanced Raman Spectroscopy with Improved Sensitivity Enabled by a Water Immersion Objective.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) appears as a promising in situ nanospectroscopic tool for characterization and understanding of the electrochemical interfacial processes at the nanometer scale and molecular level. However, the wide application of EC-TERS is hampered by its low sensitivity as a result of the optical path distortion due to the refractive index mismatch of the multilayer media (air, glass, and electrolyte). Here, we propose a new side-illumination EC-TERS setup by coupling a water immersion objective with a high numerical aperture to a scanning tunneling microscope scanning head customized with a large open space and a compact spectroelectrochemical cell. It not only effectively eliminates the optical distortion but also increases the sensitivity remarkably, which allows sensitive monitoring of the electrochemical redox processes of anthraquinone molecules. More importantly, EC-TERS is able to independently control the tip position and laser illumination position. By utilizing this feature, we reveal that the irreversible reduction reaction of anthraquinone observed in EC-TERS is induced by the synergistic effect of the negative potential and laser illumination rather than the localized surface plasmon. The highly improved sensitivity and the flexibility to control the tip and laser illumination position on the nanometer scale endows EC-TERS as an important tool for the fundamental understanding of the photo- or plasmon electrochemistry and the interfacial structure-activity relationship of important electrochemical systems.

A sensitive, low noise, DC to 12 MHz, large area photodiode preamplifier for photothermal heterodyne imaging.

We report a compact, fast, and low-noise large area photodiode preamplifier designed for photothermal heterodyne imaging (PHI). The preamplifier exhibits a noise level of 7 nV/Hz1/2 and a bandwidth from DC to 12 MHz sufficient for PHI experiments. Simulations of the preamplifier bandwidth and noise agree with the observed experimental characterization and performance in our home-built PHI system. The observed noise is close to the inherent limitations of the SR844 lock-in amplifier used. The results indicate that the preamplifier is also useful for any other single frequency pump-probe detection technique, such as stimulated Raman scattering.

Rational fabrication of silver-coated AFM TERS tips with a high enhancement and long lifetime.

Tip-enhanced Raman spectroscopy (TERS), known as nanospectroscopy, has received increasing interest as it can provide nanometer spatial resolution and chemical fingerprint information of samples simultaneously. Since Ag tips are well accepted to show a higher TERS enhancement than that of gold tips, there is an urgent quest for Ag TERS tips with a high enhancement, long lifetime, and high reproducibility, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed an electrodeposition method to fabricate Ag-coated AFM TERS tips in a highly controllable and reproducible way. We investigated the influence of the electrodeposition potential and time on the morphology and radius of the tip. The radii of Ag-coated AFM tips can be rationally controlled at a few to hundreds nanometers, which allows us to systematically study the dependence of the TERS enhancement on the tip radius. The Ag-coated AFM tips show the highest TERS enhancement under 632.8 nm laser excitation and a broad localized surface plasmon resonance (LSPR) response when coupled to a Au substrate. The tips exhibit a lifetime of 13 days, which is particularly important for applications that need a long measuring time.

Elucidating Protein/Ligand Recognition with Combined Surface Plasmon Resonance and Surface Enhanced Raman Spectroscopy.

The ability to distinguish between specific and nonspecific binding is important for assessing the interactions between protein receptors and ligands. Surface plasmon resonance (SPR) spectroscopy is an advanced tool to measure binding events, yet the ability to distinguish between specific and nonspecific binding remains a limitation. To address this problem, we use SPR spectroscopy correlated with surface enhanced Raman scattering (SERS). The chemical information present in SERS spectra provides insight into the molecular interactions between functionalized nanoparticles and proteins, which are not detectable by SPR alone. Using a custom instrument with the Kretschmann configuration, we successfully demonstrate simultaneous affinity and the chemical characterization of streptavidin-functionalized gold nanoparticles (STV-NPs) binding to biotin immobilized on a gold film in both air and flowing phosphate buffered saline (PBS). The SPR performance is consistent with that of previous reports. The association constant (KA) for streptavidin/biotin and STV-NPs/biotin interactions observed (2 ± 1 × 107 M-1 and 2.4 ± 0.3 × 1010 M-1, respectively) agree with literature values and show a strong avidity effect associated with the STV-NPs. The SERS scattering from STV-NPs is excited by the surface plasmon polariton and collected from an objective lens mounted over the fluidic channel. The SERS spectra are recorded simultaneously with the SPR sensorgram, and the detected Raman bands provide chemical insight into the binding event. Multivariate curve resolution analysis of the spectra can differentiate specific from nonspecific binding. This label-free, real time, and surface sensitive detection method provides chemical information to protein/ligand binding affinity measurements.

Photothermal Microscopy of Coupled Nanostructures and the Impact of Nanoscale Heating in Surface Enhanced Raman Spectroscopy.

The optical properties of plasmonic nanoparticles are strongly dependent on interactions with other nanoparticles, which complicates analysis for systems larger than a few particles. In this work we examined heat dissipation in aggregated nanoparticles, and its influence on surface enhanced Raman scattering (SERS), through correlated photothermal heterodyne imaging, electron microscopy and SERS measurements. For dimers the per particle absorption cross-sections show evidence of interparticle coupling, however, the effects are much smaller than those for the field enhancements that are important for SERS. For larger aggregates the total absorption was observed to be simply proportional to aggregate volume. This observation allows us to model light absorption and heating in the aggregates by assuming that the particles act as independent heat sources. The heat dissipation calculations show that very high temperatures can be created at the nanoparticle surface, and that the temperature decreases with increasing thermal conductivity of the surroundings. This is in agreement with the SERS measurements that show faster signal degradation for air compared to water environments.

Tip-enhanced Raman spectroscopy for surfaces and interfaces.

Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid-liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.

Novel Electrochemical Raman Spectroscopy Enabled by Water Immersion Objective.

Electrochemical Raman spectroscopy is a powerful molecular level diagnostic technique for in situ investigation of adsorption and reactions on various material surfaces. However, there is still a big room to improve the optical path to meet the increasing request of higher detection sensitivity and spatial resolution. Herein, we proposed a novel electrochemical Raman setup based on a water immersion objective. It dramatically reduces mismatch of the refractive index in the light path. Consequently, significant improvement in detection sensitivity and spatial resolution has been achieved from both Zemax simulation and the experimental results. Furthermore, the thickness of electrolyte layer could be expanded to 2 mm without any influence on the signal collection. Such a thick electrolyte layer allows a much normal electrochemical response during the spectroelectrochemical investigations of the methanol oxidation.

Rational fabrication of a gold-coated AFM TERS tip by pulsed electrodeposition.

Reproducible fabrication of sharp gold- or silver-coated tips has become the bottleneck issue in tip-enhanced Raman spectroscopy, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed a novel method based on pulsed electrodeposition to coat a thin gold layer over atomic force microscopy (AFM) tips to produce plasmonic TERS tips with high reproducibility. We systematically investigated the influence of the deposition potential and step time on the surface roughness and sharpness. This method allows the rational control of the radii of gold-coated TERS tips from a few to hundreds of nanometers, which allows us to systematically study the dependence of the TERS enhancement on the radius of the gold-coated AFM tip. The maximum TERS enhancement was achieved for the tip radius in the range of 60-75 nm in the gap mode. The coated gold layer has a strong adhesion with the silicon tip surface, which is highly stable in water, showing the great potential for application in the aqueous environment.

Electrochemical Tip-Enhanced Raman Spectroscopy.

Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint molecular information at nanometer resolution. We developed an electrochemical tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. We obtained potential-dependent EC-TERS from the adsorbed aromatic molecule on a Au(111) surface and observed a substantial change in the molecule configuration with potential as a result of the protonation and deprotonation of the molecule. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the molecular level and nanoscale analysis of some important electrochemical systems including solar cells, lithium ion batteries, fuel cells, and corrosion.

Tip-enhanced Raman spectroscopy: tip-related issues.

After over 15 years of development, tip-enhanced Raman spectroscopy (TERS) is now facing a very important stage in its history. TERS offers high detection sensitivity down to single molecules and a high spatial resolution down to sub-nanometers, which make it an unprecedented nanoscale analytical technique offering molecular fingerprint information. The tip is the core element in TERS, as it is the only source through which to support the enhancement effect and provide the high spatial resolution. However, TERS suffers and will continue to suffer from the limited availability of TERS tips with a high enhancement, good stability, and high reproducibility. This review focuses on the tip-related issues in TERS. We first discuss the parameters that influence the enhancement and spatial resolution of TERS and the possibility to optimize the performance of a TERS system via an in-depth understanding of the enhancement mechanism. We then analyze the methods that have been developed for producing TERS tips, including vacuum-based deposition, electrochemical etching, electrodeposition, electroless deposition, and microfabrication, with discussion on the advantages and weaknesses of some important methods. We also tackle the issue of lifetime and protection protocols of TERS tips which are very important for the stability of a tip. Last, some fundamental problems and challenges are proposed, which should be addressed before this promising nanoscale characterization tool can exert its full potential. Graphical Abstract ᅟ.

Thickness-controlled synthesis of ultrathin Au sheets and surface plasmonic property.

Thickness-controlled synthesis of nanosheets of nonlayered materials is of scientific significance yet greatly underdeveloped because of the lack of controllable means of inducing anisotropic growth of 2D structures. Here we report a novel 2D template-directed synthesis of ultrathin single-crystalline Au nanosheets with well-tuned thicknesses of several to tens of nanometers, large areas (>100 µm(2)), and atomically flat surfaces. The 2D template is composed of hundred-nanometer-thick water layers sandwiched by lamellar bilayer membranes of a self-assembled nonionic surfactant, dodecylglyceryl itaconate, which appears as an iridescent solution as a result of Bragg reflection of visible light from the periodic lamellar planes. The large-area, ultrathin single-crystalline Au nanosheets enable the fabrication of plasmonic devices. For the first time, the property of surface plasmon polaritons on a patterned single-crystalline Au nanosheet was investigated, and a long propagation length approaching the theoretical expectation was found.