The detection of various polymorphs of drug materials is crucial in the pharmaceutical industry since any variations in the structure or composition might result in a different clinical response. In this article, mechanical anisotropy and stress-induced structural changes in single-crystal β-form piroxicam (an anti-inflammatory drug) have been discussed.
The changes in the mechanical properties of the (011) and (011) crystallographic planes and the related chemical variations under indentation were analyzed in detail. A Hysitron® TI 980 TriboIndenter® for mechanical characterization and a tailor-made indentation device for in-situ Raman (Figure 1) were used for observing anisotropy in mechanical behavior along (011) and (011) crystallographic planes.
Figure 1. In-situ experimental configuration and piroxicam structure
In-Situ Indentation and Raman for Pharmaceutics
Raman spectroscopy is a non-destructive chemical analysis technique extensively used for detecting variations in the crystallinity, stress state, phase transformations, and molecular orientation in materials. When Bruker’s nanomechanical test instruments are used in combination with Raman spectroscopy, in-situ analysis of mechanical properties and associated physiochemical variations can be performed.
Variations in the chemical bonding can be detected using in-situ Raman spectra recorded from the contact region. A centrosymmetric dimer of piroxicam molecules linked by two N−H•••O (3.055 Å) hydrogen bonds is the principal building block of the piroxicam crystal. Infinite corrugated two-dimensional (2D) layers parallel to the (010) plane are formed when each molecule in the dimer interacts with adjacent dimers through six C−H•••O hydrogen bonds.
The interactions between individual 2D layers arise from one C−H•••O hydrogen bond and one π•••π stacking interaction. Consequently, lower hardness on the (011) surface (Figure 2) at the time of indentation reflects that the main slip planes are along the (010) planes.
Figure 2. Hardness and modulus from indentation tests at varying loads on the (011) face and (011) face.
In-situ Raman spectra recorded at the time of indentation on the (011) face indicated a shift in the 1,334 cm−1 band, in relation to asymmetric stretching of SO2, through a normal load variation of 3–20 mN (Figure 3). During the indentation on the (011) face, the in-situ Raman spectra did not exhibit a shift in the stretching modes of SO2; on the contrary, a small peak shift was recorded at 990 cm−1, which is in agreement with the C-O stretching (Figure 4). The 990 cm−1 band shifted to 995 cm−1 at higher loads of 15 and 20 mN.
The red shift in the stretching vibration of the C-O bond relates to a pause in intermolecular interaction of O-H••O at the time of indentation. Reasonably, it can be concluded that the deformation led to a distinctive bonding re-arrangement for the (011) and (011) planes because an intra-layer interaction modification (C−H•••O interactions) was recorded for the (011) plane, when compared to the interlayer interaction modification observed for the (011) plane. Moreover, the noticed mechanical anisotropy is not entirely associated with the orientation of crystal planes since the interlayer chemical interactions are also behind the improved hardness of the (011) surface.
The outcomes illustrate the potential of the in-situ Raman indenter in gathering the chemical information and the mechanical information in real time, which can be compared to gain an in-depth knowledge of the material behavior.
Figure 3. In-situ Raman spectra on (011) face during indentation showing a shift in 1,334 cm−1 band, corresponding to SO2 asymmetric stretching, at various normal loads.
Figure 4. In-situ Raman spectra on (011) face during indentation showing a small peak at 990 cm−1 band, corresponding to C-O stretching.
Indentation-Induced Phase Transformation in Monocrystalline Silicon
Chemical and mechanical mapping of the sample of interest can be performed by using Bruker’s Hysitron TI 950 or TI 980 system in combination with Raman spectroscopy. In the past few decades, indentation-induced phase transformation in silicon is among the most extensively studied phenomena.
The potentials of the integrated indentation and Raman system are illustrated in Figure 5, where the combination of Raman mapping and high-resolution SPM imaging allows the characterization of local chemical and topographical changes. The Raman map in Figure 5 illustrates an indentation-induced phase transformation zone in monocrystalline silicon. A Raman line scan profile created across the indent demonstrated a change in diamond cubic 520 cm−1 band, where the dc peak was shifted to higher wavenumbers by the compressive stresses at the edges. Observation of r8 and bc8 phases of silicon was made in the phase transformation zone corresponding to amorphous silicon.
Figure 5. Raman spectral map on silicon indent showing the phase transformation zone and region of compressive residual stress.
Conclusions
When combined with Raman spectroscopy, Bruker’s Hysitron TI instruments allow chemical and quantitative ultra-high-speed nanomechanical property mapping in a single platform. As Bruker sustains the idea of modularity with Raman instruments, it is possible to use virtually any spectrometer and laser source with this solution. Eventually, the 2D capacitive transducer technology offered by Bruker facilitates the testing of wear and friction at the nano level, and can even be integrated with Raman mapping to assist scientists in gaining an in-depth knowledge on the interfacial phenomena, thereby allowing innovative, advanced materials or coatings to be developed.
Other Applications
- Advanced nanomaterials: Identifying quantitative thickness, chemical heterogeneity, and measurement of strain in 2D materials (mono- or multi-layered WS2, MoS2, WSe2, MoSe2, and graphene)
- Biomaterials: Non-destructive mechanical and chemical characterization of bones, tissues, and implant materials
- Semiconductors: Detection of impurities, measurement of residual stress, and pressure-induced phase transformation
- Pharmaceutics: Mechanical and chemical heterogeneity, polymorphism
- Coatings and thin films: Detection of modifications in surface mechanical and chemical property, measurement of residual stress, and failure analysis
- High temperature: Bruker’s xSol® high-temperature stage, when used in combination with Raman spectroscopy, facilitates real-time detection of temperature-induced mechanical and chemical variations
- Polymers and composites: Correlation of mechanical characteristics with bond arrangement and cross-linking, chemical structure, as well as physical state of the polymer (together with blends) including crystallinity
References
- Manimunda, P., Hintsala, E., Asif, S. et al. JOM (2017) 69: 50. https://doi.org/10.1007/s11837-016-2169-6
About Bruker Nano Surfaces
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