Recent Progress on the Characterization of Cellulose Nanomaterials by Nanoscale Infrared Spectroscopy

Characterization of emerging 2D materials after chemical functionalization

The chemical modification of 2D materials has proven a powerful tool to fine tune their properties. With this motivation, the development of new reactions has moved extremely fast. The need for speed, together with the intrinsic heterogeneity of the samples, has sometimes led to permissiveness in the purification and characterization protocols. In this review, we present the main tools available for the chemical characterization of functionalized 2D materials, and the information that can be derived from each of them. We then describe examples of chemical modification of 2D materials other than graphene, focusing on the chemical description of the products. We have intentionally selected examples where an above-average characterization effort has been carried out, yet we find some cases where further information would have been welcome. Our aim is to bring together the toolbox of techniques and practical examples on how to use them, to serve as guidelines for the full characterization of covalently modified 2D materials.

Photoinduced force microscopy as a novel method for the study of microbial nanostructures

A detailed comparison of the capabilities of electron microscopy and nano-infrared (IR) microscopy for imaging microbial nanostructures has been carried out for the first time. The surface sensitivity, chemical specificity, and non-destructive nature of spectroscopic mapping is shown to offer significant advantages over transmission electron microscopy (TEM) for the study of biological samples. As well as yielding important topographical information, the distribution of amides, lipids, and carbohydrates across cross-sections of bacterial (Escherichia coli, Staphylococcus aureus) and fungal (Candida albicans) cells was demonstrated using PiFM. The unique information derived from this new mode of spectroscopic mapping of the surface chemistry and biology of microbial cell walls and membranes, may provide new insights into fungal/bacterial cell function as well as having potential use in determining mechanisms of antimicrobial resistance, especially those targeting the cell wall.

Recent advances in qualitative and quantitative characterization of nanocellulose-reinforced nanocomposites: A review

Nanocellulose has received immense consideration owing to its valuable inherent traits and impressive physicochemical properties such as biocompatibility, thermal stability, non-toxicity, and tunable surface chemistry. These features have inspired researchers to deploy nanocellulose as nanoscale reinforcement materials for bio-based polymers. A simple yet efficient characterization method is often required to gain insights into the effectiveness of various types of nanocellulose. Despite a decade of continuous research and booming growth in scientific publications, nanocellulose research lacks a measuring tool that can characterize its features with acceptable speed and reliability. Implementing reliable characterization techniques is critical to monitor the specifications of nanocellulose alone or in the final product. Many techniques have been developed aiming to measure the nano-reinforcement mechanisms of nanocellulose in polymer composites. This review gives a full account of the scientific underpinnings of techniques that can characterize the shape and arrangement of nanocellulose. This review aims to deliver consolidated details on the properties and characteristics of nanocellulose in biopolymer composite materials to improve various structural, mechanical, barrier and thermal properties. We also present a comprehensive description of the safety features of nanocellulose before and after being loaded within biopolymeric matrices.

Cellulose Nanocrystals (CNCs) and Cellulose Nanofibers (CNFs) as Adsorbents of Heavy Metal Ions

The isolation of nanocellulose has been extensively investigated due to the growing demand for sustainable green materials. Cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), which have the same chemical composition but have different morphology, particle size, crystallinity, and other properties depending on the precursor and the synthesis method used. In comparison, CNC particles have a short rod-like shape and have smaller particle dimensions when compared to CNF particles in the form of fibers. CNC synthesis was carried out chemically (hydrolysis method), and CNF synthesis was carried out mechanically (homogenization, ball milling, and grinding), and both can be modified because they have a large surface area and are rich in hydroxyl groups. Modifications were made to increase the adsorption ability of heavy metal ions. The Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric (TG), and dynamic light scattering (DLS) can reveal the characteristics and morphology of CNCs and CNFs. The success and effectiveness of the heavy metal adsorption process are influenced by a few factors. These factors include adsorbent chemical structure changes, adsorbent surface area, the availability of active sites on the adsorbent’s surface, adsorption constants, heavy metal ionic size differences, pH, temperature, adsorbent dosage, and contact time during the adsorption process. In this review, we will discuss the characteristics of CNCs and CNFs synthesized from various precursors and methods, the modification methods, and the application of CNCs and CNFs as heavy metal ion adsorbents, which includes suitable isotherm and kinetics models and the effect of pH on the selectivity of various types of heavy metal ions.

Colorimetric response in polydiacetylene at the single domain level using hyperspectral microscopy

The structural variance of polydiacetylene (PDA) at the nanoscale level, even under the same fabrication conditions, is one of the origins of its poor reproducibility in chemo/biosensing. In this work, we present a spatial map of such structural distributions within a single crystal by taking advantage of the recent development of hyperspectral microscopy at visible wavelengths. Hyperspectral microscopy provides the distribution of absorption spectra at the spatial resolution of standard optical microscopy. By tracking the blue-to-red transition via this technique, we found that heat or pH stimulation leaves a unique pattern in the transition pathways.

Origin of thiocyanate spectral shifts in water and organic solvents

Vibrational spectroscopy is a useful technique for probing chemical environments. The development of models that can reproduce the spectra of nitriles and azides is valuable because these probes are uniquely suited for investigating complex systems. Empirical vibrational spectroscopic maps are commonly employed to obtain the instantaneous vibrational frequencies during molecular dynamics simulations but often fail to adequately describe the behavior of these probes, especially in its transferability to a diverse range of environments. In this paper, we demonstrate several reasons for the difficulty in constructing a general-purpose vibrational map for methyl thiocyanate (MeSCN), a model for cyanylated biological probes. In particular, we found that electrostatics alone are not a sufficient metric to categorize the environments of different solvents, and the dominant features in intermolecular interactions in the energy landscape vary from solvent to solvent. Consequently, common vibrational mapping schemes do not cover all essential interaction terms adequately, especially in the treatment of van der Waals interactions. Quantum vibrational perturbation (QVP) theory, along with a combined quantum mechanical and molecular mechanical potential for solute-solvent interactions, is an alternative and efficient modeling technique, which is compared in this paper, to yield spectroscopic results in good agreement with experimental FTIR. QVP has been used to analyze the computational data, revealing the shortcomings of the vibrational maps for MeSCN in different solvents. The results indicate that insights from QVP analysis can be used to enhance the transferability of vibrational maps in future studies.

Advanced Nanocellulose-Based Materials: Production, Properties, and Applications

Natural polymers, such as polysaccharides and proteins, are being extensively utilized as substrates to create advanced materials [...].

Super-resolution mid-infrared spectro-microscopy of biological applications through tapping mode and peak force tapping mode atomic force microscope

Small biomolecules at the subcellular level are building blocks for the manifestation of complex biological activities. However, non-intrusive in situ investigation of biological systems has been long daunted by the low spatial resolution and poor sensitivity of conventional light microscopies. Traditional infrared (IR) spectro-microscopy can enable label-free visualization of chemical bonds without extrinsic labeling but is still bound by Abbe's diffraction limit. This review article introduces a way to bypass the optical diffraction limit and improve the sensitivity for mid-IR methods - using tip-enhanced light nearfield in atomic force microscopy (AFM) operated in tapping and peak force tapping modes. Working principles of well-established scattering-type scanning near-field optical microscopy (s-SNOM) and two relatively new techniques, namely, photo-induced force microscopy (PiFM) and peak force infrared (PFIR) microscopy, will be briefly presented. With ∼ 10-20 nm spatial resolution and monolayer sensitivity, their recent applications in revealing nanoscale chemical heterogeneities in a wide range of biological systems, including biomolecules, cells, tissues, and biomaterials, will be reviewed and discussed. We also envision several future improvements of AFM-based tapping and peak force tapping mode nano-IR methods that permit them to better serve as a versatile platform for uncovering biological mechanisms at the fundamental level.

Supervised Learning for Predictive Pore Size Classification of Regenerated Cellulose Membranes Based on Atomic Force Microscopy Measurements

Nanoporous dialysis membranes made of regenerated cellulose are used as molecular weight cutoff standards in bioseparations. In this study, mesoporous standards with Stokes' radii (50 kDa/2.7 nm, 100 kDa/3.4 nm and 1000 kDa/7.3 nm) and overlapping skewed distributions were characterized using AFM, with the specific aim of generating pore size classifiers for biomimetic membranes using supervised learning. Gamma transformation was used prior to conducting discriminant analysis in terms of the area under the receiver operating curve (AUC) and classification accuracy (Acc). Monte Carlo simulations were run to generate datasets (n = 10) on which logistic regression was conducted using a constant ratio of 80:20 (measurement:algorithm training), followed by algorithm validation by WEKA. The proposed algorithm can classify the 1000 kDa vs. 100 kDa (AUC > 0.8) correctly, but discrimination is weak for the 100 kDa vs. 50 kDa (AUC < 0.7), the latter being attributed to the instrument accuracy errors below 5 nm. As indicated by the results of the cross-validation study, a test size equivalent to 70% (AUC_tapping = 0.8341 ± 0.0519, Acc_tapping = 76.8% ± 5.9%) and 80% (AUC_fluid = 0.7614 ± 0.0314, Acc_tfluid = 76.2% ± 1.0%) of the training sets for the tapping and fluid modes are needed for correct classification, resulting in predicted reduction of scan times.

Carbon Dots: Classification, Properties, Synthesis, Characterization, and Applications in Health Care-An Updated Review (2018-2021)

Carbon dots (CDs) are usually smaller than 10 nm in size, and are meticulously formulated and recently introduced nanomaterials, among the other types of carbon-based nanomaterials. They have gained significant attention and an incredible interest in the field of nanotechnology and biomedical science, which is merely due to their considerable and exclusive attributes; including their enhanced electron transferability, photobleaching and photo-blinking effects, high photoluminescent quantum yield, fluorescence property, resistance to photo-decomposition, increased electrocatalytic activity, good aqueous solubility, excellent biocompatibility, long-term chemical stability, cost-effectiveness, negligible toxicity, and acquaintance of large effective surface area-to-volume ratio. CDs can be readily functionalized owing to the abundant functional groups on their surfaces, and they also exhibit remarkable sensing features such as specific, selective, and multiplex detectability. In addition, the physico-chemical characteristics of CDs can be easily tunable based on their intended usage or application. In this comprehensive review article, we mainly discuss the classification of CDs, their ideal properties, their general synthesis approaches, and primary characterization techniques. More importantly, we update the readers about the recent trends of CDs in health care applications (viz., their substantial and prominent role in the area of electrochemical and optical biosensing, bioimaging, drug/gene delivery, as well as in photodynamic/photothermal therapy).