Low-intensity phase-selective laser induced breakdown spectroscopy (PS-LIBS) with secondary resonant excitation from the same single laser pulse is employed in the in situ study of flame synthesis of TiO₂ nanoparticles. Excitation from the third harmonic (i.e. 354.71 nm as measured) of an injection-seeded Nd:YAG laser breaks down flame-synthesized titanium-dioxide nanoparticles in situ into their elements and then subsequently resonantly excites the titanium electrons. With low-intensity laser excitation (∼30 mJ per pulse or 2.4 GW cm⁻²), only the nanoparticle phase is selectively broken down, forming nano-plasmas without any observable Bremsstrahlung radiation. The induced emission at 497.534 nm resulting from the secondary resonant Ti excitation is markedly stronger than other/primary emissions. Compared to 532 nm excitation (with no secondary resonant excitation), where atomic emission intensity saturates at a laser power of ∼20 mJ per pulse (1.6 GW cm⁻² irradiance), the emissions from 354.71 nm excitation exhibit no saturation with laser irradiance because of the secondary resonant excitation, until gas-phase breakdown occurs at a laser power of 40 mJ per pulse (3.2 GW cm⁻² irradiance). Temporal evolutions of the emissions are also studied, revealing that the 497.534 nm emission maximizes earlier, but is shorter lived. Interestingly, transient line splitting with dependence on excitation laser wavelength is observed and investigated. Additional experiments scanning the excitation laser wavelength around other resonant excitation lines show that the emission intensity can be enhanced by about 130 times at 282.347 nm excitation. Thus, using the secondary resonant excitation effect, detection thresholds could be remarkably improved for phase-selective measurements of nano-aerosols.

Polydimethylsiloxane (PDMS) has emerged as a promising candidate for the dielectric layer in implantable sensors due to its exceptional biocompatibility, stability, and flexibility. This study introduces an innovative approach to produce graphene-reinforced PDMS (Gr-PDMS), where graphite powders are exfoliated into mono- and few-layer graphene sheets within the polymer solution, concurrently forming cross-linkages with PDMS. This method yields a uniformly distributed graphene within the polymer matrix with improved interfaces between graphene and PDMS, significantly reducing the percolation threshold of graphene dispersed in PDMS from 10% to 5%. As-synthesized Gr-PDMS exhibits improved mechanical and electrical properties, tested for potential use in capacitive pressure sensors. The results demonstrate an impressive pressure sensitivity up to 0.0273 kpa⁻¹, 45 times higher than that of pristine PDMS and 2.5 times higher than the reported literature value. The Gr-PDMS showcases excellent pressure sensing ability and stability, fulfilling the requirements for implantable intracranial pressure (ICP) sensors.

Effective methods are needed to fabricate the next generation of high-performance graphene-reinforced polymer matrix composites (G-PMCs). In this work, a versatile and fundamental process is demonstrated to produce high-quality graphene-polymethylmethacrylate (G-PMMA) composites via in situ shear exfoliation of well-crystallized graphite particles loaded in highly-viscous liquid PMMA/acetone solutions into graphene nanoflakes using a concentric-cylinder shearing device. Unlike other methods where graphene is added externally to the polymer and mixed, our technique is a single step process where as-exfoliated graphene can bond directly with the polymer with no contamination/handling. The setup also allows for the investigation of the rheology of exfoliation and dispersion, providing process understanding in the attainment of the subsequently heat injection-molded and solidified G-PMC, essential for future manufacturing scalability, optimization, and repeatability. High PMMA/acetone concentration correlates to high mixture viscosity, which at large strain rates results in very-high shear stresses, producing a large number of mechanically-exfoliated flakes, as confirmed by liquid-phase UV–visible spectral analysis. Raman spectroscopy and other imaging evince that single- and bi-layer graphene are readily achieved. Nevertheless, a limit is reached at high mixtures viscosities where the process becomes unstable as non-Newtonian fluid behavior (e.g. viscoelastic) dominates the system. Characterization of microstructure, morphology, and properties of this new class of nanostructured composites reveals interesting trends. Observations by transmission electron microscopy, scanning electron microscopy, and helium ion microscopy of the manufactured G-PMCs show uniform distributions of unadulterated, well-bonded, discontinuous, graphene nanoflakes in a PMMA matrix, which enhances stiffness and strength via a load-transfer mechanism. Elastic modulus of 5.193 GPa and hardness of 0.265 GPa are achieved through processing at 0.7 g ml ⁻¹ of acetone/PMMA for 1% wt. starting graphite loading when injected into a sample mold at 200 °C. Mechanical properties exhibit 31% and 28.6% enhancement in elastic modulus and hardness, respectively, as measured by nano-indentation.