Rheological insights on the evolution of sonicated cellulose nanocrystal dispersions

Cellulose nanocrystals (CNCs) are promising biomaterials, but their tendency to agglomerate when dried limits their use in several applications. Ultrasonication is commonly used to disperse CNCs in water, bringing enough energy to the suspension to break agglomerates. While the optimized parameters for sonication are now well defined for small volumes of low concentration CNC suspensions, a deeper understanding of the influence of the dispersing process is needed to work with larger volumes, at higher concentrations. Herein, rheology is used to define the distribution and dispersion states upon ultrasonication of a 3.2 wt% CNC suspension. After considering the importance of the measurement sampling volume, the behavior of a more concentrated suspension (6.4 wt%) is examined and compared with a never-dried suspension of the same concentration to validate the dispersion state.     

Evidence-based guidelines for the ultrasonic dispersion of cellulose nanocrystals

Nanoparticles possess unique, size-driven properties. However, they can be challenging to use as they easily agglomerate - their high surface area-to-volume ratio induces strong interparticle forces, generating agglomerates that are difficult to break. This issue prevails in organic particles as well, such as cellulose nanocrystals (CNCs); when in their dried form, strong hydrogen bonding enhances agglomeration. Ultrasonication is widely applied to prepare CNC suspensions, but the methodology employed is non-standardized and typically under-reported, and process efficiency is unknown. This limits the ability to adapt dispersion protocols at industrial scales. Herein, numerical simulations are used in conjunction with validation experiments to define and optimize key parameters for ultrasonic dispersion of CNCs, allowing an operating window to be inferred.     

Sensitive Raman spectroscopy of lipids based on drop deposition using DCDR and SERS

Raman spectroscopy is widely used for study of lipids and membrane models. A severe limitation of this technique lies in the low Raman cross section requiring high sample concentrations. We report sensitive detection of synthetic 1,2‐dimyristoyl‐3‐trimethylammonium‐propane (DMTAP) lipid employing two Raman techniques with improved sensitivity: drop coating deposition Raman (DCDR) and surface‐enhanced Raman scattering (SERS) spectroscopies. DCDR provided well‐reproducible DMTAP spectra without considerable loss of its solution properties if measured from the ‘coffee ring’ pattern of a drop dried on a SpectRIM^TM plate. DMTAP was detected at ~10 μM initial solution concentration, which is about three orders of magnitude lower than that for conventional Raman spectroscopy. Moreover, SERS spectra from dried ring of Ag hydrosol/DMTAP system were obtained down to ~0.3 μM DMTAP concentration, which means that sensitivity of SERS is about five orders of magnitude higher than that of conventional Raman spectroscopy. In contrast to the DCDR technique, good SERS spectra of DMTAP were obtained only from some spots of the ring containing big nanoparticle aggregates, and the structural properties of DMTAP were significantly perturbed by adsorption on the Ag nanoparticles. Copyright © 2013 John Wiley & Sons, Ltd.     

Approximate chemical analysis of volcanic glasses using Raman spectroscopy

The effect of chemical composition on the Raman spectra of a series of natural calcalkaline silicate glasses has been quantified by performing electron microprobe analyses and obtaining Raman spectra on glassy filaments (~450 µm) derived from a magma mingling experiment. The results provide a robust compositionally‐dependent database for the Raman spectra of natural silicate glasses along the calcalkaline series. An empirical model based on both the acquired Raman spectra and an ideal mixing equation between calcalkaline basaltic and rhyolitic end‐members is constructed enabling the estimation of the chemical composition and degree of polymerization of silicate glasses using Raman spectra. The model is relatively insensitive to acquisition conditions and has been validated using the MPI‐DING geochemical standard glasses 1 as well as further samples. The methods and model developed here offer several advantages compared with other analytical and spectroscopic methods such as infrared spectroscopy, X‐ray fluorescence spectroscopy, electron and ion microprobe analyses, inasmuch as Raman spectroscopy can be performed with a high spatial resolution (1 µm²) without the need for any sample preparation as a nondestructive technique. This study represents an advance in efforts to provide the first database of Raman spectra for natural silicate glasses and yields a new approach for the treatment of Raman spectra, which allows us to extract approximate information about the chemical composition of natural silicate glasses using Raman spectroscopy. We anticipate its application in handheld in situ terrestrial field studies of silicate glasses under extreme conditions (e.g. extraterrestrial and submarine environments). © 2015 The Authors Journal of Raman Spectroscopy Published by John Wiley & Sons Ltd     

Estimation of spectra sample size for characterizing single cells using micro‐Raman spectroscopy

Single‐cell micro‐Raman spectroscopy has the potential to become a powerful, new cytometric approach for discriminating between cell types and identifying subpopulations of cells based on differences in the intrinsic, molecular content of the cells. Despite the considerable progress in demonstrating many biomedical applications of single‐cell Raman spectroscopy, an unresolved issue with this method is the inconsistent manner in which the technique has been applied experimentally to acquire spectra from a cell, which can potentially lead to irreproducible and inconsistent results from study to study. Specifically, the problem pertains to the validity of using micro‐Raman spectroscopy to sample an arbitrary fraction of the cell volume and to claim that the resulting spectrum represents the entire cell volume. In this study, we investigate the effect of sample size (i.e. the number of Raman spectra acquired from distinct locations in a cell) on the ability to generate a Raman spectrum that accurately describes the total molecular content of the cell. Furthermore, we demonstrate that a minimum sample size of Raman spectra acquired from a cell can be used in place of a full hyperspectral Raman image to achieve the same degree of discrimination between different cell populations. Lymphocytes exposed to the chemotherapy drug, doxorubicin, at different concentrations and exposure times are used as a model biological system in this study. This work demonstrates the importance of adequate spectral sampling and presents an approach for determining the minimum sample size needed to reproduce a Raman spectrum of a whole cell, which are expected to impact future single‐cell Raman spectroscopy studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Clay‐induced Preferred Orientation in Polyethylene/Compatibilized Clay Nanocomposites

Nanocomposites of the organically modified clay Cloisite^® 15A (CL15A) dispersed in HDPE‐g‐MA were prepared by melt‐compounding. Microcomposites of the same clay with HDPE were also obtained with similar procedures. The spherulitic morphology of the polymer matrix was evidenced by optical microscopy in thin films, whereas the structure of the up to 2‐mm–thick, compression‐molded samples was investigated by WAXD and SAXS. Preferred orientation of both the clay and the HDPE crystallites were evidenced in the microcomposites and, to a greater extent, in nanocomposites, whereas in HDPE and HDPE‐g‐MA control specimens hardly any anisotropy was detected. The degree of orientation of PE crystals increases with CL15A concentration, but also with clay exfoliation, with lower cooling rates and decreasing sample thickness. The orientation of the clay platelets parallel to the compression‐molded surface appears to be determined by the platelets anisotropy and by shear in the mixing and the compression‐molding procedures. In turn, it determines the preferred uniaxial orientation of HDPE crystals, which have their crystallographic a axis orthogonal, while b and c are coplanar, to the sample surface, as already reported in the literature for melt‐crystallized HDPE films with thickness below 0.3 μm. It is proposed that the HDPE orientation results from confined crystallization between parallel clay platelets which are on average less than 0.1 μm apart. Simple models, qualitatively accounting for the observed orientation of HDPE, are discussed. Organized architectures resulting from confined crystallization of the polymer matrix in nanocomposites with appropriate anisotropic fillers may be a general feature, important in determining key properties of these systems.     

Ultraviolet tip‐enhanced nanoscale Raman imaging

We have constructed an ultraviolet (UV)‐apertureless near‐field scanning optical microscope‐Raman spectroscopy system by using an aluminum tip for the simultaneous measurement of topography and Raman scattering of nanomaterials with high spatial resolution. The topography, Rayleigh scattering image, and tip‐enhanced Raman scattering image of the carbon nanotube film showed that a spatial resolution of around 19 nm was achieved. This spatial resolution of UV‐Raman mapping image exceeds that of previous approaches, which have several hundred nanometers of spatial resolution. Copyright © 2012 John Wiley & Sons, Ltd.     

Drop‐coating deposition Raman spectroscopy of liposomes

Drop‐coating deposition Raman (DCDR) spectroscopy was tested as a potential technique for studying liposomes at very low sample concentrations. We used model liposomes prepared either from 1,2‐distearoyl‐sn‐glycero‐3‐phospocholine or from soybean asolectin, which is composed of various lipids and thus represents a good model of natural membranes. In both cases, deposited samples formed a dried drop with a circular shape with a ring of concentrated liposomes at the edge. Spectral mapping showed that maximum Raman intensity originated from the inner part of the edge ring, while Raman signal gradually decreased in both radial directions. The Raman spectra exhibited excellent reproducibility of spectral characteristics at different locations in the drop, indicating similar conformation and ordering of hydrocarbon lipid chains in the sample. Our results suggest that DCDR spectroscopy can be used for studying lipids in situ, and sensitivity of this technique is at least two orders of magnitude higher than that of conventional Raman microscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

The influence of impurity formation on electron inelastic scattering of suspended graphene

This study reports on controlling the formation of nanoimpurities on suspended graphene to investigate the inelastic scattering of electrons using a two‐phonon Raman process. Results were analyzed by transmission electron microscopy (TEM) and scanning Raman spectroscopy in the same region of suspended graphene. The findings revealed that the area with a higher concentration of impurities shown in the TEM image corresponds directly to the area with a lower integrated intensity and a wider full width at half maximum in the Raman mapping of the 2D band and vice versa. The same trend is also apparent in the 2D′ and D + D″ bands. In conclusion, the results are explained by an increase in the electronic scattering rate due to impurities, which affects two‐phonon Raman scattering. Combining the TEM image and Raman mapping image effectively demonstrates how electron behavior is affected by the distribution of impurities in graphene systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Assembly‐Induced Emission of Cellulose Nanocrystals for Hiding Information

Fluorescent‐labeled cellulose nanocrystal (CNC) films have been used to record and protect information in paper materials, whereas the fluorescent materials usually suffer photobleaching. Herein, a strategy of solid‐state emission induced by the vertical assembly of CNCs was established. The assembly‐induced emission starts from the structural diffraction of CNC, whose wavelength is adjusted into the ultraviolet (UV) region for hiding information under natural light. The small diameter (≈10 nm) of CNCs then promotes the resonance between the vertically assembled CNCs and the UV light, leading to a strong blue emission with an emitting quantum efficiency as high as 13.90%. By introducing the vertical‐assembly film with a specific pattern into paper materials, an anti‐counterfeiting image is obtained under a UV radiation. Since CNCs are a kind of cellulose with high crystallinity, this material can be a wear‐resistant anti‐counterfeiting material for banknotes or other paper applications.     

Tribological Properties of Polycarbonate Blended with High‐density Polyethylene and High‐density Polyethylene Grafted with Maleic Anhydride

High‐density polyethylene (HDPE) and maleic anhydride grafted HDPE (HDPE‐g‐MA) were selected as lubricant and compatibilizer, respectively, for improving the tribological and mechanical properties of polycarbonate (PC). The morphology of worn surfaces and debris was observed by means of scanning electron microscopy (SEM). The mated steel ring surface was analyzed by using SEM combined with energy dispersive spectroscopy (EDS). Both HDPE and HDPE‐g‐MA reduced the friction and wear of pure PC. HDPE‐g‐MA, which had a better compatibility with PC than HDPE, resulted in better improvement of the mechanical and tribological properties of the PC matrix. A 10 vol. % HDPE‐g‐MA reduced the wear of pure PC by 4 orders of magnitude, and the friction coefficient was reduced from 0.86 to 0.22. Such improvements in the tribological behavior resulted from the good self lubrication of HDPE and HDPE‐g‐MA. The PC/HDPE‐g‐MA (S_90‐0‐10) polyblend also showed higher notched impact strength than pure PC. It may be a useful material for application in tribological fields.     

Identification of Raman spectra through a case‐based reasoning system: application to artistic pigments

This paper presents a methodology conceived as a support system to identify unknown materials by means of the automatic recognition of their Raman spectra. Initially, the design and implementation of the system were framed in an artistic context where the Raman spectra analyzed belong to artistic pigments. The analysis of the pigmentation used in an artwork constitutes one of the most important contributions in its global study. This paper proposes a methodology to systematically identify Raman spectra, following the way analysts usually work in their laboratory but avoiding their assessment and subjectivity. It is a three‐phase methodology that automates the spectral comparison, which is based on one of the most powerful paradigms inmachine learning: the case‐based reasoning (CBR) systems. A CBR system is able to solve a problem by using specific knowledge of previous experiences (well‐known spectral library of patterns) and finding the most similar past cases (patterns), reusing and adapting them to the new problem situation (unknown spectrum). The system results in a global signal processing methodology that includes different phases such as reducing the Raman spectral expression by means of the principal component analysis, the definition of similarity measures to objectively quantify the spectral similarity and providing a final value obtained by a fuzzy logic system that will help the analyst to take a decision. The major benefit of a Raman spectral identification system lies in offering a decision‐support tool to those who are not experts or under difficult situations with respect to Raman spectroscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

Monitoring by Raman spectroscopy of the damage induced in the wake of energetic ions

This article reports micro‐Raman experiments performed on cross sections of 6H‐SiC crystals irradiated with heavy ions of different energies. The results demonstrate that this technique is very powerful to quantify the damage created in the wake of energetic ions from the surface of samples down to the ion resting position. For slow ions (900‐keV I), ballistic collisions lead to the amorphization of the surface region of samples. For swift ions (36‐MeV W), the surface region remains crystalline and amorphization occurs around the end of the ion path. Moreover, synergistic effects between electronic and nuclear slowing down processes are put forward. The methodology used in this work may be adapted to other materials where radiation effects need to be investigated, provided that the damage created by irradiation is detectable by Raman spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.     

Comparison of chemometric methods in the analysis of pharmaceuticals with hyperspectral Raman imaging

Chemical imaging method of vibrational spectroscopy, which provides both spectral and spatial information, creates a three‐dimensional (3D) dataset with a huge amount of data. When the components of the sample are unknown or their reference spectra are not available, the classical least squares (CLS) method cannot be applied to create visualized distribution maps. Raman image datasets can be evaluated even in such cases using multivariate (chemometric) methods for extracting the needed hidden information. The capability of chemometrics‐assisted Raman mapping is evaluated through the analysis of pharmaceutical tablets (considered as unknown) with the aim of estimating the pure component spectra based on the collected Raman image. Six chemometric methods, namely, principal component analysis (PCA), maximum autocorrelation factors (MAF), sample–sample 2D correlation spectroscopy (SS2D), self‐modeling mixture analysis (SMMA), multivariate curve resolution–alternating least squares (MCR‐ALS), and positive matrix factorization (PMF), were compared. SMMA was found to be the best choice to determine the number of components. MCR‐ALS and PMF provided the pure component spectra with the highest quality. MCR‐ALS was found to be superior to PMF in the estimation of Raman scores (which correspond to the concentrations) and yielded almost the same results as CLS (using the real reference spectra). Thus, the combination of Raman mapping and chemometrics could be successfully used to characterize unknown pharmaceuticals, identify their ingredients, and obtain information about their structures. This may be useful in the struggles against illegal and counterfeit products and also in the field of pharmaceutical industry when contaminants are to be identified. Copyright © 2011 John Wiley & Sons, Ltd.     

Non‐invasive depth profiling by space and time‐resolved Raman spectroscopy

Time‐resolved Raman spectroscopy, spatially offset Raman spectroscopy and time‐resolved spatially offset Raman spectroscopy (TR‐SORS) have proven their capability for the non‐invasive profiling of deep layers of a sample. Recent studies have indicated that TR‐SORS exhibits an enhanced selectivity toward the deep layers of a sample. However, the enhanced depth profiling efficiency of TR‐SORS, in comparison with time‐resolved Raman spectroscopy and spatially offset Raman spectroscopy, is yet to be assessed and explained in accordance to the synergistic effects of spatial and temporal resolutions. This study provides a critical investigation of the depth profiling efficiency of the three deep Raman techniques. The study compares the efficiency of the various deep Raman spectroscopy techniques for the stand‐off detection of explosive precursors hidden in highly fluorescing packaging. The study explains for the first time the synergistic effects of spatial and temporal resolutions in the deep Raman techniques and their impact on the acquired spectral data. Copyright © 2013 John Wiley & Sons, Ltd.     

Discrimination of carotenoid and flavonoid content in petals of pansy cultivars (Viola x wittrockiana) by FT‐Raman spectroscopy

Fourier Transform Raman spectroscopy (FT‐Raman) has been applied for the non‐destructive in‐situ analysis of pigments on differently colored flower petals of pansy cultivars (Viola x wittrockiana). The main target of the present study was to investigate how far the Raman mapping technique through FT‐Raman spectroscopy and cluster analysis of the Raman spectra is a potential method for the direct, in‐situ discrimination of flavonoids (flavonols against anthocyanins) and of carotenoids occurring in flowers, using intact and differently colored flower petal of Viola x wittrockiana for this case study. In order to get more information about the reliability of the direct in‐situ flavonoid detection by the Raman method, pigments extracts of the petals were separated by thin‐layer chromatography (TLC) and investigated by Raman spectroscopy. Hierarchical cluster analysis (HCA) of the Raman spectra from reference pigments (carotenoids, anthocyanins and flavonols), from areas of the flower petals, and from the TLC extracts allowed discriminating the various pigments, in particular flavonoids (flavonols against anthocyanins) and carotenoids. With a two‐dimensional Raman mapping technique, which provides a chemical image of the sample under investigation, we determined by cluster analysis the distribution of carotenoids, anthocyanins and flavonols from the outer layer of the petals, and by integrating through suitable spectral regions selected as characteristic markers for particular pigments their relative concentration could approximately be determined. We found a satisfactory correlation between the patterns seen on the visible images and the patterns on the chemical images obtained by Raman mapping. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of both fingerprint and high wavenumber Raman spectroscopy of pathological nasopharyngeal tissues

High wavenumber (HW) Raman spectroscopy has weaker fluorescence background compared with fingerprint (FP) region. This study aims to evaluate the discrimination feasibility of nasopharyngeal non‐cancerous and nasopharyngeal cancer (NPC) tissue with both FP and HW Raman spectroscopy. HW Raman spectra of nasopharyngeal tissue were obtained for the first time. Raman spectra were collected to differentiate nasopharyngeal non‐cancerous (n = 37) from NPC (n = 41) tissues in FP (800–1800cm⁻¹), HW (2700–3100cm⁻¹), and integrated FP/HW region. First, to assess the utility of this method, the averaged Raman spectral intensities and intensity ratios of corresponding Raman bands were analyzed in HW and FP regions, respectively. The results show that intensities as well as the ratios of specific Raman peaks might be helpful in distinguishing nasopharyngeal non‐cancerous from NPC tissue with the HW Raman spectroscopy, as with FP Raman reported before. The multivariate statistical method based on the combination of principal component analysis–liner discriminant analysis (PCA‐LDA), together with leave‐one‐patient‐out, cross‐validation diagnostic algorithm, was used for discriminating nasopharyngeal non‐cancerous from NPC tissue, generating sensitivities of 87.8%, 85.4%, and 95.1% and specificities of 86.5%, 91.9%, and 89.2%, respectively, with Raman spectroscopy in the FP, HW, and integrated FP/HW regions. The posterior probability of classification results and receiver operating characteristic curves were utilized to evaluate the discrimination of PCA‐LDA algorithm, verifying that HW Raman spectroscopy has a positive effect on the differentiation for the diagnosis of NPC tissue by integrated FP/HW Raman spectroscopy. What's more, the potential of Raman spectroscopy used for differentiating different pathology NPC tissues was also discussed. The results demonstrate that both FP and HW Raman spectroscopy have the potential for diagnosis and detection in early nasopharyngeal carcinoma, and HW Raman spectroscopy may improve the discrimination of NPC tissue compared with FP region alone, providing a promising diagnostic tool for the diagnosis of NPC tissue. Copyright © 2015 John Wiley & Sons, Ltd.     

Infrared versus light scattering techniques to monitor the gel to liquid crystal phase transition in lipid membranes

In this study, Fourier transform infrared, Raman and Brillouin spectroscopy have been used to study lipid phase behavior of hydrated as well as dried multilamellar l ‐α‐phosphatidylcholine assemblies, in order to compare limitations and potentials of the different techniques. Dried lipid samples have been studied in the presence and absence of trehalose, which is known to affect the phase behavior of these systems. The methylene C‐H stretching (2800–3000 cm⁻¹) region in infrared (IR) and Raman spectra provided mutually consistent information on the rearrangement of lipid acyl chains occurring at the lipid melting temperature. IR spectra have a higher signal‐to‐noise ratio, thus permitting a more precise evaluation of the melting temperature. In the hydrated lipid samples, the C‐H stretching region in the Raman spectra is less affected by the contribution of water compared with that in the IR spectra. Raman spectra are particularly suitable to simultaneously study both lipid and water contributions allowing to distinguish ice from non‐frozen water below 0 °C. Brillouin light scattering was used to probe the collective dynamics, i.e. the propagation velocity and the attenuation of longitudinal acoustic modes in the lipid samples. Lipid phase transitions are evident from a change in the temperature behavior of the acoustic velocity. Moreover, a strong relaxation process with a characteristic time of 14 ps was observed in the sample dried without trehalose with a maximum in acoustic attenuation at about 45 °C, which likely reflects the rearrangement of acyl chains. Copyright © 2015 John Wiley & Sons, Ltd.     

General criterion to discern between coherent and incoherent synthesis of broadband coherent anti‐Stokes Raman spectra

Recently, the ordinary qualitative criterion on how to distinguish between coherent and incoherent convolutions of broadband coherent anti‐Stokes Raman (CARS) signals generated by degenerate pump lasers has been revised in view of a quantitative analysis. The revision has established that incoherent CARS approach can be justified as unitary limit of the function ] erfc(Γ/σ₁)/σ₁, where Γ and σ₁ are respectively the spectral widths of the Raman line and the degenerate pump lasers. The result was, however, limited to nonoverlapping Raman lines. In this work, the extension to a more common situation of closely spaced Raman transitions is considered. For large overlap between adjacent Raman lines, the new analysis suggests significant deviations from the previous result. Weak line mixing is also taken into consideration. Nonetheless, all types of deviations are characterized by a common tendency toward the incoherent limit. Copyright © 2007 John Wiley & Sons, Ltd.     

The quality control of two Pueraria species using Raman spectroscopy coupled with partial least squares analysis

The dried roots of Pueraria lobata (Puerariae Lobatae Radix; PLR) and Pueraria thomsonii (Puerariae Thomsonii Radix; PTR) are medicinal herbs that are used interchangeably in clinical practice, even though their chemical profiles are different. Therefore, the aim of this study was to develop a rapid and non‐destructive method for the quality control of Pueraria species using Raman spectroscopy in combination with partial least squares analysis. Partial least squares‐discriminant analysis (PLS‐DA) was used to differentiate PLR from PTR, whereas partial least squares regression (PLSR) was used to predict the total phenolic content (TPC) and antioxidant capacities of the Pueraria species. Raman spectroscopy revealed that spectral characteristics of starch and polyphenols differentiated the two species, with the PLS‐DA model giving 100% classification accuracy for the tested samples. A significantly higher TPC (p < 0.001), 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) (ABTS) radical scavenging activity (p < 0.001) and cupric reducing antioxidant capacity (CUPRAC; p < 0.001) were observed for PLR as compared to PTR. The high ratio of performance to deviation values (TPC: 9.84; ABTS: 7.11; CUPRAC: 7.13) indicated the PLSR models were robust for predicting TPC and antioxidant capacities. The loading plot revealed that the content of starch and polyphenols were important factors in differentiating PLR from PTR and predicting TPC and antioxidant capacities. The results demonstrate that Raman spectroscopy coupled with chemometrics is a rapid method for the quality control of PLR and PTR. These methods can be applied as a template for the quality control of other herbal medicines and products to promote the correct identification of herbs for clinical practice. Copyright © 2015 John Wiley & Sons, Ltd.