Optical trapping has become an important noninvasive tool to manipulate and immobilize microscopic particles. In particular, plasmonic tweezers provide sufficient gradient force with much lower laser powers that are capable of trapping subwavelength particles. However, the expensive and time-consuming fabrication techniques used in making plasmonic nanostructures often hinders direct application of such optical tweezers. Here, we demonstrate a novel trapping scheme using the optical and electronic properties of graphene oxide layers. First, we demonstrate the trapping of polystyrene beads on the graphene oxide-based substrate with a multimode laser illumination at very low intensity. We then present the trapping of silica-shelled quantum dots on graphene oxide layers thereby allowing the study of the quenching behavior of quantum dots on graphene oxide. The technique is then extended to live biological specimens (Escherichia coli bacteria) wherein bacteria is trapped and immobilized on graphene oxide in a noninvasive way. This scheme will be useful for studying biomolecular processes such as cell metabolism, cytotoxicity, and cell stimuli. This system will also be an inexpensive but effective replacement of plasmonically enhanced optical tweezers.

Optically-assisted large-scale assembly of nanoparticles have been of recent interest owing to their potential in applications to assemble and manipulate colloidal particles and biological entities. In the recent years, plasmonic heating has been the most popular mechanism to achieve temperature hotspots needed for extended assembly and aggregation. In this work, we present an alternative route to achieving strong thermal gradients that can lead to non-equilibrium transport and assembly of matter. We utilize the excellent photothermal properties of graphene oxide to form a large-scale assembly of silica beads. The formation of the assembly using this scheme is rapid and reversible. Our experiments show that it is possible to aggregate silica beads (average size 385 nm) by illuminating thin graphene oxide microplatelet by a 785 nm laser at low intensities of the order of 50–100 µW/µm2. We further extend the study to trapping and photoablation of E. coli bacteria using graphene oxide. We attribute this aggregation process to optically driven thermophoretic forces. This scheme of large-scale assembly is promising for the study of assembly of matter under non-equilibrium processes, rapid concentration tool for spectroscopic studies such as surface-enhanced Raman scattering and for biological applications.

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The study of generation, growth and decay of microbubbles is interesting owing to its potential applications in imaging, trapping of colloidal particles and mass fluid flow. While there have been several reports on the generation of microbubbles using plasmonic nanostructure based substrates, they often are expensive and cumbersome to fabricate. In this study, we demonstrate a simple scheme for the generation of microbubbles using graphene oxide microstructures. Due to the excellent photothermal properties of this graphene-based 2D material, it is possible to generate and sustain microbubbles by laser illumination at low intensities of the order of few of W/m2. The size of the microbubbles can be tuned by changing the incident laser power. Furthermore, the generated microbubble acts as a concave wide-angle lens with variable focal length. We further utilised the fluid flow around the microbubble to generate large-scale assembly of silica beads and quantum dots using thermophoretic forces. This optically-assisted thermophoretic aggregation is rapid and reversible. Owing to the simple and cost-effective synthesis method of graphene oxide, this scheme is an excellent alternative to plasmonic heating based- methods for the generation of microbubbles