Nanothermal Analysis Research Articles

Recent Advances in Solid-State Analysis of Pharmaceuticals

Current analytical techniques for characterizing solid-state pharmaceuticals include powder x-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, infrared spectroscopy, Raman spectroscopy, electron microscopy and nuclear magnetic resonance. Powder x-ray diffraction and differential scanning calorimetry are mainstream techniques but they lack spatial resolution. Scanning electron microscopy and micro-Raman spectroscopy provide good chemical and optical characterization but they are not capable of analysing very small nanoparticles. Transmission electron microscopy and nano-thermal analysis can provide explicit characterization of nanoparticles but they are invasive. Nuclear magnetic resonance offers good spatial resolution but its use is mainly limited by poor sensitivity and high costs. In view of the many challenges posed by existing methods, new and novel techniques are being continually researched and developed to cater to the growing number of solid formulations in the pipeline and in the market. Some of the recent advances attained in the solid-state analysis of pharmaceutical are summarized in this review article.

Co-registered Topographical, Band Excitation Nanomechanical, and Mass Spectral Imaging Using a Combined Atomic Force Microscopy/Mass Spectrometry Platform.

The advancement of a hybrid atomic force microscopy/mass spectrometry imaging platform demonstrating the co-registered topographical, band excitation nanomechanical, and mass spectral imaging of a surface using a single instrument is reported. The mass spectrometry-based chemical imaging component of the system utilized nanothermal analysis probes for pyrolytic surface sampling followed by atmospheric pressure chemical ionization of the gas-phase species produced with subsequent mass analysis. The basic instrumental setup and operation are discussed, and the multimodal imaging capability and utility are demonstrated using a phase-separated polystyrene/poly(2-vinylpyridine) polymer blend thin film. The topography and band excitation images showed that the valley and plateau regions of the thin film surface were comprised primarily of one of the two polymers in the blend with the mass spectral chemical image used to definitively identify the polymers at the different locations. Data point pixel size for the topography (390 nm × 390 nm), band excitation (781 nm × 781 nm), and mass spectrometry (690 nm × 500 nm) images was comparable and submicrometer in all three cases, but the data voxel size for each of the three images was dramatically different. The topography image was uniquely a surface measurement, whereas the band excitation image included information from an estimated 20 nm deep into the sample and the mass spectral image from 110 to 140 nm in depth. Because of this dramatic sampling depth variance, some differences in the band excitation and mass spectrometry chemical images were observed and were interpreted to indicate the presence of a buried interface in the sample. The spatial resolution of the chemical image was estimated to be between 1.5 and 2.6 μm, based on the ability to distinguish surface features in that image that were also observed in the other images.

The Heat Resistance of Microbial Cells Represented by D Values Can be Estimated by the Transition Temperature and the Coefficient of Linear Expansion.

We previously developed a method for evaluating the heat resistance of microorganisms by measuring the transition temperature at which the coefficient of linear expansion of a cell changes. Here, we performed heat resistance measurements using a scanning probe microscope with a nano thermal analysis system. The microorganisms studied included six strains of the genus Bacillus or related genera, one strain each of the thermophilic obligate anaerobic bacterial genera Thermoanaerobacter and Moorella, two strains of heat-resistant mold, two strains of non-sporulating bacteria, and one strain of yeast. Both vegetative cells and spores were evaluated. The transition temperature at which the coefficient of linear expansion due to heating changed from a positive value to a negative value correlated strongly with the heat resistance of the microorganism as estimated from the D value. The microorganisms with greater heat resistance exhibited higher transition temperatures. There was also a strong negative correlation between the coefficient of linear expansion and heat resistance in bacteria and yeast, such that microorganisms with greater heat resistance showed lower coefficients of linear expansion. These findings suggest that our method could be useful for evaluating the heat resistance of microorganisms.

水晶発振子マイクロバランス法によるポリアミド樹脂へのトリエトキシシリルトリアジンジチオールの吸着挙動とその被膜の局所熱分析

スピンコートによってポリアミド樹脂被覆水晶発振子センサーセル（PA6/QC）を作製し，水晶発振子マイクロバランス（QCM）を用いてPA6/QCへの6-［［3-（triethoxysilyl）propyl］amino］-1,3,5-triazine-2,4-dithiol mono sodium（TES）の吸着挙動に及ぼすTESの濃度，Buffer溶液のpHの影響を検討した。そして局所熱分析によってPA6表面に形成したTES被膜の転移温度について検討した。TESの分子構造は計算から高さ1.2 nm，幅0.8 nm，厚さ0.6 nmとなり，TES一個当たりの占有面積は4.8×10−13 mm2となった。QCMの結果から0.1 M TES水溶液でBuffer溶液pH 3のとき1649 ng/cm2となりTES吸着量が最大となった。局所熱分析によってPA6表面に吸着したTESがプローブの昇温によって100℃付近でシラノールが脱水縮合してシロキサン結合を形成することで，TESシロキサン被膜の熱安定性が230℃であることがわかった。

Atomic Force Microscope Controlled Topographical Imaging and Proximal Probe Thermal Desorption/Ionization Mass Spectrometry Imaging

This paper reports on the development of a hybrid atmospheric pressure atomic force microscopy/mass spectrometry imaging system utilizing nanothermal analysis probes for thermal desorption surface sampling with subsequent atmospheric pressure chemical ionization and mass analysis. The basic instrumental setup and the general operation of the system were discussed, and optimized performance metrics were presented. The ability to correlate topographic images of a surface with atomic force microscopy and a mass spectral chemical image of the same surface, utilizing the same probe without moving the sample from the system, was demonstrated. Co-registered mass spectral chemical images and atomic force microscopy topographical images were obtained from inked patterns on paper as well as from a living bacterial colony on an agar gel. Spatial resolution of the topography images based on pixel size (0.2 μm × 0.8 μm) was better than the resolution of the mass spectral images (2.5 μm × 2.0 μm), which were limited by current mass spectral data acquisition rate and system detection levels.

Direct Measurement of Chain Diffusion at Interfaces of PPO/PS Bilayer Films by Nano-Thermal Analysis and Time-of-Flight Secondary Ion Mass Spectrometry

Chain diffusion at the polyphenylene oxide/polystyrene (PPO/PS) interface of bilayer films were investigated using nanothermal analysis (nano-TA) and time-of-flight secondary ion mass spectrometry (TOF–SIMS). An ultralow-angle slicing technique by the surface and interfacial cutting analysis system (SAICAS) was used to expand the size of the interfacial region effectively. The glass-transition temperature (Tg) at the interface was directly evaluated by nano-TA. The gradient of Tg at the PPO/PS interface became broad after annealing. An increase in annealing time caused an increase in the Tg of the initial PS layer, indicating that the PPO diffused across the interface. Since PPO/PS is a compatible blend system, the local chemical compositions were evaluated by Fox equation. The methodology presented here is useful for interfacial analyses of various polymer composite systems and is beneficial because the isotope labeling is not necessary.

Stabilisation of amorphous drugs under high humidity using pharmaceutical thin films

In this study, the stabilization effects of three polymers on four model drugs (felodipine, fenofibrate, carbamazepine, and celecoxib) under saturated humidity were investigated. Three different types of thin films (solid dispersions, drug films with a polymer film coating and drug films laid on top of polymer coated surfaces) were prepared and compared with films containing the drug alone. ATR-FTIR spectroscopy, polarised light microscopy (PLM), scanning electron microscopy (SEM) and nano-thermal analysis (nano-TA) were performed on the model systems after storage under saturated humidity. The recrystallisation tendency of the drug in the drug containing thin films was found to be strongly related to the intrinsic crystallization tendency of the drug film alone and the strength of drug–polymer interactions. Additionally, under high humidity, the glass transition temperature of the polymer is no longer an indicator of its drug stabilization capability. Instead, it is the hygroscobicity of the polymer that appears to be the most important parameter. Amongst the polymers tested in this study, EUDRAGIT E PO was found to have the greatest inhibitory effect on crystallization, whilst PVP K30 was found to have the least protective effect; presumably because of its hygroscopic nature.

Early Stage Phase Separation in Pharmaceutical Solid Dispersion Thin Films under High Humidity: Improved Spatial Understanding Using Probe-Based Thermal and Spectroscopic Nanocharacterization Methods

Phase separation in pharmaceutical solid dispersion thin films under high humidity is still poorly understood on the submicrometer scale. This study investigated the phase separation of a model solid dispersion thin film, felodipine-PVP K29/32, prepared by spin-coating and analyzed using probe-based methods including atomic force microscopy, nanothermal analysis, and photothermal infrared microspectroscopy. The combined use of these techniques revealed that the phase separation process occurring in the thin films under high humidity is different from that in dry conditions reported previously. The initial stage of phase separation is primarily initiated in the bulk of the films as amorphous drug domains. Drug migration toward the surface of the solid dispersion film was then observed to occur under exposure to increased humidity. PVP cannot prevent phase separation of felodipine under high humidity but can minimize the crystallization of amorphous felodipine domains in the solid dispersion thin films. This study demonstrates the unique abilities of these nanocharacterization methods for studying, in three dimensions, the phase separation of thin films for pharmaceutical applications.

With respect to coefficient of linear thermal expansion, bacterial vegetative cells and spores resemble plastics and metals, respectively

BackgroundIf a fixed stress is applied to the three-dimensional z-axis of a solid material, followed by heating, the amount of thermal expansion increases according to a fixed coefficient of thermal expansion. When expansion is plotted against temperature, the transition temperature at which the physical properties of the material change is at the apex of the curve. The composition of a microbial cell depends on the species and condition of the cell; consequently, the rate of thermal expansion and the transition temperature also depend on the species and condition of the cell. We have developed a method for measuring the coefficient of thermal expansion and the transition temperature of cells using a nano thermal analysis system in order to study the physical nature of the cells.ResultsThe tendency was seen that among vegetative cells, the Gram-negative Escherichia coli and Pseudomonas aeruginosa have higher coefficients of linear expansion and lower transition temperatures than the Gram-positive Staphylococcus aureus and Bacillus subtilis. On the other hand, spores, which have low water content, overall showed lower coefficients of linear expansion and higher transition temperatures than vegetative cells. Comparing these trends to non-microbial materials, vegetative cells showed phenomenon similar to plastics and spores showed behaviour similar to metals with regards to the coefficient of liner thermal expansion.ConclusionsWe show that vegetative cells occur phenomenon of similar to plastics and spores to metals with regard to the coefficient of liner thermal expansion. Cells may be characterized by the coefficient of linear expansion as a physical index; the coefficient of linear expansion may also characterize cells structurally since it relates to volumetric changes, surface area changes, the degree of expansion of water contained within the cell, and the intensity of the internal stress on the cellular membrane. The coefficient of linear expansion holds promise as a new index for furthering the understanding of the characteristics of cells. It is likely to be a powerful tool for investigating changes in the rate of expansion and also in understanding the physical properties of cells.

Evaluation of the crystalline and amorphous states of drug products by nanothermal analysis and Raman imaging

In recent years, amorphous formulations and other special dosage forms of drug products have been investigated to achieve adequate solubility and disintegration. We have evaluated the distribution of crystalline and amorphous states of a drug product using Nanothermal analysis (Nano-TA) and Raman imaging methods. Compared to conventional differential scanning calorimetry, Nano-TA can be used to more rapidly characterize the crystalline and amorphous states of model formulations, including their ingredient distributions, without any sample preparation. In the current study, imaging maps obtained for specific model formulations were evaluated on the basis of their visual appearance and the physicochemical properties of the active pharmaceutical ingredient (API). In addition, the crystalline and amorphous states of the model formulations were distinguished by Raman mapping. Nano-TA was found to be useful for the characterization of crystalline and amorphous states of APIs and the distribution of other ingredients. This technology could be used to monitor the changes in crystalline forms of drug substances and dosage forms during processing. In addition, Nano-TA can be used to characterize amorphous states.

Nanoscale Surface Characterization and Miscibility Study of a Spray-Dried Injectable Polymeric Matrix Consisting of Poly(lactic-co-glycolic acid) and Polyvinylpyrrolidone

Injectable controlled-release formulations are of increasing interest for the treatment of chronic diseases. This study aims to develop and characterize a polymeric matrix for intramuscular or subcutaneous injection, consisting of two biocompatible polymers, particularly suitable for formulating poorly soluble drugs. For this matrix, the water-insoluble polymer poly(lactic-co-glycolic acid) (PLGA) is combined with the water-soluble polymer polyvinylpyrrolidone (PVP). Microparticles of these two polymers were prepared by spray drying. The phase behavior of the samples was studied by means of modulated differential scanning calorimetry and the results showed that phase separation occurred in the bulk sample through evidence of two mixed amorphous phases, namely, a PLGA-rich phase and a PVP-rich phase. Characterization of the samples by scanning electron microscopy demonstrated that the spray-dried particles were hollow with a thin shell. Because of the importance in relation to stability and drug release, information about the surface of the microparticles was collected by different complementary surface analysis techniques. Atomic force microscopy gathered information about the morphology and phase behavior of the microparticle surface. Time-of-flight secondary ion mass spectrometry analysis of the particles revealed that the surface consisted mainly of the PLGA-rich phase. This was confirmed by X-ray photoelectron spectroscopy at an increased sampling depth (≈ 10 nm). Nanothermal analysis proved to be an innovative way to thermally detect the presence of the PLGA-dominated surface layer and the underlying PVP phase. Taken together, this information provides a rational basis for predicting the likely drug release behavior this formulation will display.

The Development of Thermal Nanoprobe Methods as a Means of Characterizing and Mapping Plasticizer Incorporation into Ethylcellulose Films

The phase composition and distribution of ethylcellulose (EC) films containing varying amounts of the plasticizer fractionated coconut oil (FCO) were studied using a novel combination of thermal and mapping approaches. The thermal and thermomechanical properties of films containing up to 30% FCO were characterized using modulated temperature differential scanning calorimetry (MTDSC) and dynamic mechanical analysis (DMA). Film surfaces were mapped using atomic force microscopy (AFM; topographic and pulsed force modes) and the composition of specific regions identified using nanothermal probes. Clear evidence of distinct conjugate phases was obtained for the 20-30% FCO/EC film systems. We suggest a model whereby the composition of the distinct phases may be estimated via consideration of the glass transition temperatures observed using DSC and DMA. By combining pulsed force AFM and nano-thermal analysis we demonstrate that it is possible to map the two separated phases. In particular, the use of thermal probes allowed identification of the distinct regions via localized thermomechanical analysis, whereby nanoscale probe penetration is measured as a function of temperature. The study has indicated that by using thermal and imaging techniques in conjunction it is possible to both identify and map distinct regions in binary films.

Polystyrene-based blend nanorods with gradient composition distribution

The polystyrene-based polymer blends, partially miscible poly(bisphenol A carbonate)/polystyrene (PC/PS) and completely miscible poly(2,6-dimethylphenylene oxide)/polystyrene (PPO/PS), in nanorods with gradient composition distribution were discussed. The polymer blend nanorods were prepared by infiltrating the polymer blends into nanopores of anodic aluminum oxide (AAO) templates via capillary action. Their morphology was investigated by micro-Fourier transform infrared spectroscopy (micro-FTIR) and nano-thermal analysis (nano-TA) with spatial resolution. The composition gradient of polymer blends in the nanopores is governed by the difference of viscosity and miscibility between the two polymers in the blends and the pore diameter. The capillary wetting of porous AAO templates by polymer blends offers a unique method to fabricate functional nanostructured materials with gradient composition distribution for the potential application to nanodevices.

Surface Modification of Polyethylene Terephthalate (PET) by 172-nm Excimer Lamp

We studied the effects of 172 nm Xe2* excimer lamp irradiation on polyethylene terephthalate (PET) surfaces. Two kinds of techniques were applied: vacuum ultraviolet (VUV) light irradiation and VUV irradiation in the presence of oxygen gas (VUV/O3). The modified PET surfaces were investigated by using contact angle measurements which enabled the surface free energy to be calculated, X-ray photoelectron spectroscopy (XPS), nano-thermal analysis (nano-TA), and atomic force microscopy (AFM). The surface free energy increased significantly after the treatments. The results of XPS analysis showed that the elemental ratio of oxygen on the surface increased, whereas that of carbon decreased. From the deconvoluted C1s and O1s spectra, it was revealed that new oxidized functional groups such as alcoholic and carboxyl groups were generated. The nano-TA results showed that a low melting temperature (Tm) layer had formed on the VUV and VUV/O3 treated PET surfaces. The results of AFM measurements showed there were no remarkable changes after the treatments compared with untreated PET. In summary, the VUV and VUV/O3 treatments using a Xe2* excimer lamp not only change the surface functionalities but also reduce the Tm of the PET surfaces without significantly affecting the surface morphologies.

Erratum to: Potential of AFM–nanothermal analysis to study the microscale thermal characteristics in soils and natural organic matter (NOM)

Erratum to: Potential of AFM–nanothermal analysis to study the microscale thermal characteristics in soils and natural organic matter (NOM)

Potential of AFM–nanothermal analysis to study the microscale thermal characteristics in soils and natural organic matter (NOM)

This exploratory study evaluates the potential of nanothermal analysis (nTA) coupled with atomic force microscopy (AFM) of soil samples for understanding physicochemical processes in soil and for linking the nanospatial and microspatial distribution of thermal characteristics with the macroscopic properties of soil samples. Soil and reference samples were investigated by differential scanning calorimetry and AFM-nTA. nTA was conducted on 16 points of each AFM image in two subsequent heating cycles (55–120°C and 55–300°C, respectively). Thermograms were subdivided into characteristic types and their spatial distribution was compared between sample replicates and materials. Thermogram types consisted of partly structured expansion and compression phases, suggesting material-specific thermal profiles. The distribution of thermogram types reflected sample-dependent nanoscale and microscale heterogeneity. Indications for water molecule bridge transitions were found by nTA in peat and soil. Organic materials generally revealed strong expansion and irreversible compression phases, latter probably due to the collapse of pore and aggregate structures. In contrast to charcoal and manure, peat shows strong expansion below 120°C and compression only above 120°C. All investigated samples are heterogeneous on the nanoscale and microscale with respect to thermal behaviour. AFM-nTA allows distinguishing numerous different materials on nanometre and micrometre scales in soil samples. The material-dependent characteristics will help in understanding and learning more about the nanoscale distribution of different materials and properties. Related to the macroscopic thermal behaviour, this will allow studying links between the properties of biogeochemical interfaces and the processes governed by them.

Inkjet printing as a novel medicine formulation technique

We demonstrate the viability of using an ink-jet printer to produce a formulation capable of controlling the release of a drug. This is shown for the drug felodipine, an antihypertensive, with polyvinyl pirrolidone (PVP) as an excipient. As felodipine is a poorly water soluble drug, its molecular dispersion in a soluble polymer (ie. PVP) is a commonly used approach to improve bioavailability. Various ratios of felodipine and PVP in an ethanol-DMSO mixture (95/5) were dispensed in picoliter quantities using a piezoelectric ‘ink-jet’ head onto a hydrophobic substrate. The resultant formulation spots were characterized using atomic force microscopy, localized nano-thermal analysis, ATR-IR and imaging confocal Raman spectroscopy. Intimate mixing of the felodipine and PVP within the micro-dots was observed. ATR-IR confirmed the known molecular level interaction of felodipine and PVP through hydrogen bonding. Nanothermal analysis indicated a single glass transition point, indicative of an intimate polymer drug mixture, which is lowered as the drug concentration increases. Confocal Raman microscopy mapping on single micro-scale droplets allowed the visualization of the drug distribution in the spots as well as facilitating characterization of the release of the drug. The drug release can be altered through control of the drug loading. As inkjet printing is an inherently scalable technology, this proof of principal work with single deposited micro-spot formulations demonstrates the potential of this approach to print practical dosage forms (e.g. as an array of many thousands of spots with different release profiles). This, for example, raises the possibility in the future of producing dosage forms at points of care with one or more drugs which have been formulated for the needs of individual patients.

Studies on low-temperature direct bonding of VUV/O3-, VUV- and O2 plasma-pre-treated poly-methylmethacrylate

Low-temperature direct bonding of poly-methylmethacrylate (PMMA) plates was achieved by pre-treatment with vacuum ultraviolet irradiation in the presence of oxygen gas (VUV/O3), with vacuum ultraviolet irradiation in a nitrogen atmosphere (VUV), or with oxygen plasma. Based on surface analysis by attenuated total reflection Fourier-transform infrared spectrometry (ATR-FT-IR), x-ray photoelectron spectroscopy and near edge x-ray absorption fine structure spectroscopy, chemical changes of the PMMA surface after the pre-treatment were investigated. Changes in morphology and softening point were also investigated by nano-thermal analysis and atomic force microscopy. From the results, the bonding mechanisms proposed are described as follows: during the bonding process, the increased or generated polar groups cause an increase in the dipolar interactions (such as hydrogen bonding) between two pre-treated PMMA surfaces; in the case of VUV/O3 or VUV pre-treatment, a low-Tg layer is generated on the surface and this layer acts as an adhesive for the direct bonding of layers by diffusion.

Combined Atomic Force Microscope-Based Topographical Imaging and Nanometer-Scale Resolved Proximal Probe Thermal Desorption/Electrospray Ionization–Mass Spectrometry

Nanometer- scale proximal probe thermal desorption/electrospray ionization mass spectrometry (TD/ESI-MS) was demonstrated for molecular surface sampling of caffeine from a thin film using a 30 nm diameter nanothermal analysis (nano-TA) probe tip in an atomic force microscope (AFM) coupled via a vapor transfer line and ESI interface to a MS detection platform. Using a probe temperature of 350 °C and a spot sampling time of 30 s, conical desorption craters 250 nm in diameter and 100 nm deep were created as shown through subsequent topographical imaging of the surface within the same system. Automated sampling of a 5 × 2 array of spots, with 2 μm spacing between spots, and real time selective detection of the desorbed caffeine using tandem mass spectrometry was also demonstrated. Estimated from the crater volume (∼2 × 10(6) nm(3)), only about 10 amol (2 fg) of caffeine was liberated from each thermal desorption crater in the thin film. These results illustrate a relatively simple experimental setup and means to acquire in an automated fashion submicrometer scale spatial sampling resolution and mass spectral detection of materials amenable to TD. The ability to achieve MS-based chemical imaging with 250 nm scale spatial resolution with this system is anticipated.

Compositional Analysis of Low Quantities of Phase Separation in Hot-Melt-Extruded Solid Dispersions: A Combined Atomic Force Microscopy, Photothermal Fourier-Transform Infrared Microspectroscopy, and Localised Thermal Analysis Approach

To characterise phase separations in aged hot-melt-extruded solid dispersions at a micron to submicron scale. Hot-melt-extruded felodipine and Eudragit® E PO systems at a range of compositions were studied after a standard period of aging to allow phase separation to occur. The samples were characterised using combined nano-thermal analysis, photothermal FTIR microspectroscopy coupled with pulsed force mode AFM as a novel characterisation approach. Crystalline felodipine presents in all formulations with drug loadings from 10-70% (w/w). In formulations with high drug loadings (50 and 70%), amorphous felodipine co-exists with crystalline forms, and higher drug concentration is observed in the centre compared to the outer surface of the extrudates. Drug crystal dimensions in extrudates with low drug loadings (10-30%) are small, in the micron to submicron range. We propose that uneven drug distribution is principally caused by processing-associated factors such as expansion of extrudates during extrusion. We have demonstrated that the novel combined approach allows site-specific characterisation of the extruded systems and that drug distribution may be uneven across the extrudates, with concomitant implications for understanding stability and drug release behaviour.