Analytical Chemistry最新文献

Nanoscale zinc-oxide doped with aluminum ZnO:Al is studied by different techniques targeting surface changes induced by the conditions at which ZnO:Al is used as support material in the catalysis of methanol. While it is well established that a variety of ¹H and ²⁷Al resonances can be found by solid-state NMR for this material, it was not clear yet which signals are related to species located close to the surface of the material and which to species located in the bulk. To this end, a method is suggested that makes use of a paramagnetically impregnated material to suppress NMR signals close to the particle surface in the blind sphere around the paramagnetic metal atoms. It is shown that it is important to use conditions that guarantee a stable reference system relative to which it can be established whether the coating procedure is conserving the original structure or not. This method, called paramagnetically assisted surface peak assignment, helped to assign the ¹H and ²⁷Al NMR peaks to the bulk and the surface layer defined by the blind sphere of the paramagnetic atoms. The assignment results are further corroborated by the results from heteronuclear ²⁷Al{¹H} dipolar dephasing experiments, which indicate that the hydrogen atoms are preferentially located in the surface layer and not in the particle core.

It is well-known that the bacterial microenvironment imposes restrictions on the growth and behavior of bacteria. The localized monitoring of microenvironmental factors is appreciated when consulting bacterial adaptation and behavior in the presence of chemical or mechanical stimuli. Herein, we developed a novel liquid crystal (LC) biosensor in a microsphere configuration for real-time 3D monitoring of the bacteria microenvironment, which was implemented by a microfluidic chip. As a proof of concept, a LC gel (LC-Gel) microsphere biosensor was prepared and employed in the localized pH changes of bacteria by observing the configuration change of LC under polarized optical microscopy. Briefly, the microsphere biosensor was constructed in core-shell configuration, wherein the core contained LCE7 (a nematic LC) doped with 4-pentylbiphenyl-4'-carboxylic acid (PBA), and the shell encapsulated the bacteria. The protonation of carboxyl functional groups of the PBA induced a change in charge density on the surface of LCE7 and the orientation of E7 molecules, resulting in the transitions of the LC nucleus from axial to bipolar. The developed LC-Gel microspheres pH sensor exhibited its dominant performance on localized pH real-time sensing with a resolution of 0.1. An intriguing observation from the prepared pH biosensor was that the diverse bacteria impelled distinct acidifying or alkalizing effects. Overall, the facile LC-Gel microsphere biosensor not only provides a versatile tool for label-free, localized pH monitoring but also opens avenues for investigating the effects of chemical and mechanical stimuli on cellular metabolism within bacterial microenvironments.

Strongly confined electric fields resulting from nanogaps within nanoparticle aggregates give rise to significant enhancement of surface-enhanced Raman scattering (SERS). Nanometer differences in gap sizes lead to drastically different confined field strengths; so much attention has been focused on the development and understanding of nanostructures with controlled gap sizes. In this work, we report a novel petal gap-enhanced Raman tag (GERT) consisting of a bipyramid core and a nitrothiophenol (NTP) spacer to support the growth of hundreds of small petals and compare its SERS emission and localization to a traditional bipyramid aggregate. To do this, we use super resolution spectral SERS imaging that simultaneously captures the SERS images and spectra while varying the incident laser polarization. Intensity fluctuations inherent of SERS enabled super resolution algorithms to be applied, which revealed subdiffraction limited differences in the localization with respect to polarization direction for both particles. Interestingly, however, only the traditional bipyramid aggregates experienced a strong polarization dependence in their SERS intensity and in the plasmon-induced conversion of NTP to dimercaptoazobenzene (DMAB), which was localized with nanometer precision to regions of intense electromagnetic fields. The lack of polarization dependence (validated through electromagnetic simulations) and surface reactions from the bipyramid-GERTs suggests that the emissions arising from the bipyramid-GERTs are less influenced by confined fields.

Human-borne acetone is a potent marker of lipid metabolism. Here, an enzyme immobilization method for secondary alcohol dehydrogenase (S-ADH), which is suitable for highly sensitive and selective biosensing of acetone, was developed, and then its applicability was demonstrated for spatiotemporal imaging of concentration distribution. After various investigations, S-ADH-immobilized meshes could be prepared with less than 5% variation by cross-linking S-ADH with glutaraldehyde on a cotton mesh at 40 °C for 15 min. Furthermore, high activity was obtained by adjusting the concentration of the coenzyme nicotinamide adenine dinucleotide (NADH) solution added to the S-ADH-immobilized mesh to 500 μM and the solvent to a potassium phosphate buffer solution at pH 6.5. The gas imaging system using the S-ADH-immobilized mesh was able to image the decrease in NADH fluorescence (ex 340 nm, fl 490 nm) caused by the catalytic reaction of S-ADH and the acetone distribution in the concentration range of 0.1-10 ppm-v, including the breath concentration of healthy people at rest. The exhaled breath of two healthy subjects at 6 h of fasting was quantified as 377 and 673 ppb-v, which were consistent with the values quantified by gas chromatography-mass spectrometry.

The growing interest in lignin valorization in the past decades calls for analytical techniques for lignin characterization, ranging from wet chemistry techniques to highly sophisticated chromatographic and spectroscopic methods. One of the key parameters to consider is the molecular weight profile of lignin, which is routinely determined by size-exclusion chromatography; however, this is by no means straightforward and is prone to being hampered by considerable errors. Our study expands the fundamental understanding of the bias-inducing mechanisms in gel permeation chromatography (GPC), the magnitude of error originating from using polystyrene standards for mass calibration, and an evaluation of the effects of the solvent and type of lignin on the observed bias. The developed partial least-squares (PLS) regression model for lignin-related monomers revealed that lignin is prone to association mainly via hydrogen bonding. This hypothesis was supported by functional group-based analysis of the bias as well as pulse field gradient (pfg) diffusion NMR spectroscopy of model compounds in THF-d₈. Furthermore, although the lack of standards hindered drawing conclusions based on functionalities, direct infusion electrospray ionization mass spectrometry indicated that the relative bias decreases considerably for higher molecular weight species. The results from pfg-diffusion NMR spectroscopy on whole lignin samples were comparable when the same solvents were used in both experiments; in addition, the comparison between results obtained by pfg-diffusion NMR in different solvents gives some additional insights into the aggregation.

Exploring the ability of four-stranded DNA nanorings (fsDNRs) to host multiple nanosilver clusters (NAgCs) for cooperatively amplifiable fluorescence biosensing to a specific initiator (tI*) is fascinating. By designing three DNA single strands and three analogous stem-loop hairpins, we developed a functional fsDNR through sequential cross-opening and overlapped hybridization. Note that a substrate strand (SS) was programmed with six modules: two severed splits (sT and sT') of NAgCs template, two sequestered segments by a middle unpaired spacer, and a partition for tI*-recognizable displacement, while sT and sT' were also tethered in two ends of three hairpins. At first, a triple dsDNA complex with stimulus-responsiveness was formed to guide the specific binding to tI*, while the exposed toehold of the SS activated the forward cascade hybridization of three hairpins, until the ring closure in the tailored self-assembly pathway for forming the fsDNR. The resulting four duplexes forced each pair of sT/sT' to be merged as the parent template in four nicks, guiding the preferential synthesis of four clusters in the shared fsDNR, thereby cooperatively amplifying the green fluorescence signal for sensitive assay of tI*. Meanwhile, the topological conformation of fsDNR can be stabilized by the as-formed cluster adducts to rivet the pair of two splits in the nicks. Benefitting from the self-enhanced effect of multiple emitters, this label-free fluorescent sensing strategy features simplicity, rapidity, and high on-off contrast, without involving complicated nucleic acid amplifiers.

The elucidation of protein-membrane interactions is pivotal for comprehending the mechanisms underlying diverse biological phenomena and membrane-related diseases. In this investigation, vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy, utilizing synchrotron radiation (SR), was employed to dynamically observe membrane interaction processes involving water-soluble proteins at the secondary-structure level. The study utilized a time-resolved (TR) T-shaped microfluidic cell, facilitating the rapid and efficient mixing of protein and membrane solutions. This system was instrumental in acquiring measurements of the time-resolved circular dichroism (TRCD) spectra of β-lactoglobulin (bLG) during its interaction with lysoDMPG micelles. The results indicate that bLG undergoes a β-α conformation change, leading to the formation of the membrane-interacting state (M-state), with structural alterations occurring in more than two steps. Global fitting analysis, employing biexponential functions with all of the TRCD spectral data sets, yielded two distinct rate constants (0.18 ± 0.01 and 0.06 ± 0.003/s) and revealed a unique spectrum corresponding to an intermediate state (I-state). Secondary-structure analysis of bLG in its native (N-, I-, and M-states) highlighted that structural changes from the N- to I-states predominantly occurred in the N- and C-terminal regions, which were prominently exposed to the membrane. Meanwhile, transitions from the I- to M-states extended into the inner barrel regions of bLG. Further examination of the physical properties of α-helical segments, such as effective charge and hydrophobicity, revealed that the N- to I- and I- to M-state transitions, which are ascribed to first- and second-rate constants, respectively, are primarily driven by electrostatic and hydrophobic interactions, respectively. These findings underscore the capability of the TR-VUVCD system as a robust tool for characterizing protein-membrane interactions at the molecular level.

Mitochondrial Membrane Chromatography (MMC) is a bioaffinity chromatography technique developed to study the interaction between target proteins embedded in the mitochondrial membrane and their ligand compounds. However, the MMC stationary phases (MMSP) prepared by chemical immobilization are prone to nonspecific binding in candidate agent screening inevitably. To address these challenges, Twin Strep-Tag/Strep Tactin was employed to establish a specific affinity system in the present study. We prepared a carnitine palmitoyltransferase 1A (CPT1A) MMSP by specifically linking Strep-tactin-modified silica gel with the Twin Strep-Tag on the CPT1A-oriented mitochondrial membrane. This Twin Strep-Tag/Strep Tactin modified CPT1A/MMC method exhibited remarkably better retention behavior, longer stationary phase lifespan, and higher screening specificity compared with previous MMC systems with glutaraldehyde immobilization. We adopted the CPT1A-specific MMC system in screening CPT1A ligands from traditional Chinese medicines, and successfully identified novel candidate ligands: ononin, isoliquiritigenin, and aloe-emodin, from Glycyrrhiza uralensis Fisch and Senna tora (L.) Roxb extracts. Biological assessments illustrated that the compounds screened promote CPT1A enzyme activity without affecting CPT1A protein expression, as well as effectively reduce the lipid droplets and triglyceride levels in the high fat induction HepG2 cells. The results suggest that we have developed an MMC system, which is promising for studying the bioaffinity of mitochondrial membrane proteins to candidate compounds. This system provides a platform for a key step in mitochondrial medicine discovery, especially for bioactive molecule screening from complex herbal extracts.

Measurement of infrared spectroscopy has emerged as a significant challenge for carbon materials due to the sampling problem. To overcome this issue, in this work, we performed measurements of IR spectra for carbon materials including C₆₀, C₇₀, diamond powders, graphene, and carbon nanotubes (CNTs) using the photoacoustic spectroscopy (PAS) technique; for comparison, the vibrational patterns of these materials were also studied with a conventional transmission method, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, or Raman spectroscopy. We found that the IR photoacoustic spectroscopy (IR-PAS) scheme worked successfully for these carbon materials, offering advantages in sampling. Interestingly, the profiles of IR-PAS spectra for graphene and CNTs exhibit negative bands using carbon black as the reference; the negative spectral information may provide valuable knowledge about the storage energy, production, structure, defect, or impurity of graphene and CNTs. Thus, this approach may open a new avenue for analyzing carbon materials.