Abstract
We fabricated nanomaterials comprising amino-functionalized and nitrogen-doped graphene quantum dots (amino-N-GQDs) and investigated their photostability and intrinsic luminescence in the near-infrared spectrum to determine their suitability as contrast agents in two-photon imaging (TPI). We observed that amino-N-GQDs with a higher amount of bonded nitrogen and amino-functionalized groups (6.2%) exhibited superior two-photon properties to those with a lower amount of such nitrogen and groups (4.9%). These materials were conjugated with polymers containing sulfur (polystyrene sulfonate, PSS) and nitrogen atoms (polyethylenimine, PEI), forming amino-N-GQD–PSS–PEI specimens (amino-N-GQD-polymers). The polymers exhibited a high quantum yield, remarkable stability, and notable two-photon properties and generated no reactive oxygen species, rendering them excellent two-photon contrast agents for bioimaging. An antiepidermal growth factor receptor (AbEGFR) was used for labeling to increase specificity. Two-photon imaging (TPI) of amino-N-GQD (6.2%)-polymer-AbEGFR-treated A431 cancer cells revealed remarkable brightness, intensity, and signal-to-noise ratios for each observation at a two-photon excitation power of 16.9 nJ pixel−1 under 30 scans and a three-dimensional (3D) depth of 105 µm, indicating that amino-N-GQD (6.2%)-polymer-AbEGFR-treated cells can achieve two-photon luminescence with 71 times less power required for two-photon autofluorescence (1322.8 nJ pixel−1 with 500 scans) of similar intensity. This economy can minimize photodamage to cells, rendering amino-N-GQD-polymers suitable for noninvasive 3D bioimaging.
Highlights
Nanotechnology has been widely studied and applied in almost every field
The estimated size approximately matched that obtained from the High-resolution transmission electron microscopy (HR-TEM) calculations; the value obtained from the Raman estimation (~6.9 nm) was marginally lower than that obtained from HR-TEM because the oxygenated regions were ignored in the Raman estimation (Equations (1) and (2), Supporting Information) [13,14]
N–π * transitions of the C–N and C = O shoulder were observed at approximately 325 nm. This indicates the occurrence of a π-electron transition in the amino-N-Graphene quantum dots (GQDs) (6.2%) containing oxygen, demonstrating that the dots were doped with nitrogen, as confirmed through ultraviolet–visible (UV–Vis) spectroscopy (Figure 1C)
Summary
Nanotechnology has been widely studied and applied in almost every field. Nanotechnology is used for therapy, diagnosis, and detection. Highly stable and biocompatible carbon (C)-based nanomaterials are widely used in bioimaging. The formation of surface groups may be attributed to a new phenomenon linked to defect- and intrinsic-state emissions and to a photoluminescence (PL) mechanism [1]. Intrinsic-state emission is induced by the quantum size effect, recombination of localized electron-hole pairs, or zigzag edge sites, whereas defect-state emission results from the defect effects (energy traps). GQD-based materials exhibit a low PL quantum yield (QY), which is inadequate for practical application. This drawback substantially limits the application of GQD-based materials in photomedicine
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