Modular functionalization of crystalline graphene by recombinant proteins: a nanoplatform for probing biomolecules

Graphene, as well as other two-dimensional materials, is a promising candidate for use in bioimaging, therapeutic drug delivery, and bio-sensing applications. Here, we developed a protocol to functionalize graphene with recombinant proteins using genetically encoded SpyTag-SpyCatcher chemistry. SpyTag forms a covalent isopeptide bond with its genetically encoded partner SpyCatcher through spontaneous amidation under physiological conditions. The functionalization protocol developed is based on the use of short proteins as a linker, where two graphene-binding-peptides (GBPs) are attached to both ends of SpyTag (referred to as GStG), followed by the covalent conjugation with SpyCatcher-fusion proteins. The proposed method enables the decoration of crystalline graphene with various proteins, such as fluorescent proteins and affibody molecules that bind to cancerous cells. This scheme, which takes advantage of the cleanness of single-crystal graphene and the robustness of SpyTag-SpyCatcher chemistry, provides a versatile platform on which to study the biomolecule-surface and cell-substrate interactions and, indeed, may lead to a new way of designing biomedical devices. The interaction between peptides and graphene was clearly shown using molecular dynamics simulation and proven using specially designed experiments.

Recoil Effect and Photoemission Splitting of Trions in Monolayer MoS2

The 2D geometry nature and low dielectric constant in transition-metal dichalcogenides lead to easily formed strongly bound excitons and trions. Here, we studied the photoluminescence of van der Waals heterostructures of monolayer MoS₂ and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS₂ and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important.

Single-probe multistate detection of DNA via aggregation-induced emission on a graphene oxide platform

Graphene and graphene oxides (GO), or their reduced forms, have been introduced in a variety of biosensing platforms and have exhibited enhanced performance levels in these forms. We herein report a DNA sensing platform consisting of aggregation-induced emission (AIE) molecules and complementary DNA (comDNA) adsorbed on GO. We experimentally turned the AIE molecule on and off by adjusting its distance, which correlates with DNA structures as shown in our computational results, from the GO sheet, which quenches depending on its distance from the graphene plane. The changes in florescence are reproducible, which demonstrates the probe's ability to identify the binding state of the DNA. Our molecular dynamics simulation results reveal strong π-π interactions between single-strand DNA (ssDNA) and GO, which enable the ssDNA molecule to move closer to the graphene oxide. This reduces the center of mass and binding free energies in the simulation. When hybridized with comDNA, the increased distance, evidenced by the reduced interaction, eliminates the quenching effect and turns on the AIE molecule. Our protocol use of the AIE molecule as a probe thus avoids the complicated steps involved in covalent functionalization and allows the rapid and label-free detection of DNA molecules.

STATEMENT OF SIGNIFICANCE

A simple, rapid method of fluorescent measurement of DNA hybridization in the presence of graphene (oxide) is presented. Conventional fluorescent dyes offer high performance in biosensors. However, labeling procedures are synthetically demanding in time and resources making it less cost-effective. Molecules with aggregation-induced-emission (AIE) property have advantages over traditional fluorescent molecules because of their intrinsic preference for detection as a turn-on probe and their single-molecule detection ability. Previous work has shown AIE dyes act as excellent "label-free" bioprobes with high sensitivity but with limited selectivity. Graphene oxide (GO) with its unique optical properties and affinity to different kinds of biomolecules can be used as an auxiliary to enhance selectivity of AIE dyes. In this work, we report a label-free strategy to detect DNA of particular sequence by water-soluble AIE probes with the aid of GO, supported by the computational explanations for this phenomenon.

Oxidized nitinol substrate for interference enhanced Raman scattering of monolayer graphene

We prepared a controllable and reproducible active NiTi alloy substrate with tunable thickness of titanium oxide capping layer for surface enhanced Raman scattering (SERS) of monolayer graphene using a facile oxidation method.