Carbon nanodots (CNDs) have attracted great attention due to their superior solubility, biocompatibility, tunable photoluminescence, and opto-electronic properties. This work describes a new fluorescence-based spectroelectrochemistry approach to simultaneously study the photoluminescence and wavelength dependent photocurrent of microwave synthesized CNDs. The fluorescence of CNDs has a selective quench upon a reversible redox couple, ferricyanide/ferrocyanide,− reaction during the cyclic voltammetry. The CNDs modified gold slide electrode demonstrates wavelength dependent photocurrent generation during the fluorescence-electrochemical study, suggesting potential application in photoelectronics. UV-Vis absorption and electrochemistry are used to quantify the energy gap of the CNDs, and then to calibrate a Hückel model for the CNDs electronic energy levels. The Hückel (or tight binding) model treatment of an individual CND as a molecule combines the conjugated π states (C=C) with the functional groups (C=O, C-O, and COOH) associated with the surface electronic states. This experimental and theoretical investigation of the CNDs provides a new perspective on the optoelectronic properties of CNDs and should aid in their development for practical use in biomedicine, chemical sensing, and photoelectric devices.

Carbon nanodots (CNDs) are featured with a wide range of light absorption and excitationdependent fluorescence. The emission enhancement of CNDs is of great interest for the development of nanophotonics. Although the phenomenon of plasmon-enhanced fluorescence for quantum dots and molecular dyes has been well investigated, rarely has it been reported for CNDs. In this work, a series of plasmonic nanoslit designs were fabricated and utilized for immobilization of CNDs in nanoslits and examination of the best match for plasmonic fluorescence enhancement of CNDs. In concert, to better understand the plasmonic effect on the enhancement, the surface optical field is measured with or without CND immobilization using a hyperspectral imaging system as a comparison, and a semianalytical model is conducted for a quantitative analysis of surface plasmon generation under the plane-wave illumination. Both the fluorescence and surface reflection light intensity enhancement are demonstrated as a function of nanoslit width and are maximized at the 100 nm nanoslit width. The analysis of surface plasmon-exciton coupling of CNDs in the nanoslit area suggests that the enhancement is primarily due to plasmonic light trapping for increased electromagnetic field and plasmoninduced resonance energy transfer. This study suggests that incorporating CNDs in the plasmonic nanoslits may provide a largely enhanced CND-based photoemission system for optical applications.

A novel plasmonic nanoledge device was presented to explore the geometry-induced trapping of nanoscale biomolecules and examine a generation of surface plasmon resonance (SPR) for plasmonic sensing. To design an optimal plasmonic device, a semianalytical model was implemented for a quantitative analysis of SPR under plane-wave illumination and a finite-difference time-domain (FDTD) simulation was used to study the optical transmission and refractive index (RI) sensitivity. In addition, total internal reflection fluorescence (TIRF) imaging was used to visualize the migration of fluorescently labeled bovine serum albumin (BSA) into the nanoslits; and fluorescence correlation spectroscopy (FCS) was further used to investigate the diffusion of BSA in the nanoslits. Transmission SPR measurements of free prostate specific antigen (f-PSA), which is similar in size to BSA, were performed to validate the trapping of the molecules via specific binding reactions in the nanoledge cavities. The present study may facilitate further development of single nanomolecule detection and new nanomicrofluidic arrays for effective detection of multiple biomarkers in clinical biofluids.