Boron Doped Carbon Dots with Unusually High Photoluminescence Quantum Yield for Ratiometric Intracellular pH Sensing

Abstract

Herein we report that boron doping in carbon dots results in increased photoluminescence (PL) quantum yield, which could be used for ratiometric intracellular pH sensing in cancer cell lines. Using a mixture of citric acid monohydrate, thiourea, and boric acid, microwave‐assisted synthesis of boron doped blue emitting carbon dots (B‐Cdots) with an average size of 3.5±1.0 nm was achieved. For B‐Cdots, the maximum quantum yield (QY) was observed to be 25.8 % (11.1 % (w/w) H3BO3 input concentration), whereas, the same was calculated to be 16.9 % and 11.4 % for Cdots (synthesized from citric acid monohydrate and thiourea only) and P‐Cdots (phosphorus doped carbon dots; synthesized using citric acid monohydrate, thiourea and phosphoric acid) (11.1 % (w/w) H3PO4 input concentration), respectively. The observed luminescence efficiencies as obtained from steady state and time‐resolved photoluminescence measurements suggest an alternative emission mechanism due to boron/phosphorus doping in carbon dots. We furthermore demonstrated facile composite formation using B‐Cdots and another carbon dots with orange emission in presence of polyvinyl alcohol (PVA), resulting in white light emission (0.31, 0.32; λex 380 nm). The white light emitting composite enabled ratiometric pH sensing in the aqueous medium and showed favorable uptake properties by cancerous cells for intracellular pH sensing as well.

Comparison of the effects of synthesis methods of B, N, S, and P-doped carbon dots with high photoluminescence properties on HeLa tumor cells

“…The doped CDs were fabricated by following the latest work with some modications. 15,22,23 The bare CDs as well as boron-, nitrogen-, sulphur-and phosphor-doped CDs were prepared by combining citric acid (88.9 mg) with boric acid, nitric acid, sulphuric acid and phosphoric acid following the composition given in Table 1, respectively. For the furnace-assisted method, each mixture was calcined at 300 C for 2 h in an isolated reactor.…”

Recent advances in crystalline carbon dots for superior application potential

“…82,94 Apart from these fundamental transition bands, additional absorption bands could be observed in some Cdot systems in the longer wavelength region due to the presence of multiple surface functional groups resulting in smaller electronic band gaps. 14,95 4.1.2 Emission characteristics. Cdots are considered as an emerging class of ''nanolights'' because of their fascinating optical properties, high quantum yield, wavelength tunable emission and good photostability.…”

“…Apart from oxygen containing surface functional groups, nitrogen doping (resulting in -NH 2 , -CONH 2 , -CHNR surface functional groups) and doping with sulphur, boron, and phosphorus result in significant changes in PL QY along with tuning of the emission wavelength. 14,47,100 Fig. 3 illustrates the corresponding surface state controlled extrinsic emission in Cdots.…”

“…Low toxicity, good biocompatibility, high QY and stable optical properties make Cdots promising fluorescent probes as compared to organic dyes or semiconducting QDs. 14,96 Fluorescence based sensing techniques using Cdots can be broadly categorized into two types such as the solution phase detection and solid phase detection. The solution phase detection technique has been used to sense various analytes such as small molecules, intracellular pH, macromolecules, cations, anions and moisture content.…”

“…On the other hand, that of Cdots is yet to be fully explored. Cdots are luminescent zero-dimensional nanomaterials and have drawn tremendous attention because of their potential use in the fields of biology, [8][9][10][11][12] sensing, [13][14][15][16] catalysis, [17][18][19][20] and energy storage and conversion. [21][22][23][24] Their ease of synthesis, structural flexibility, high photoluminescence quantum yield (PLQY), optical stability and low toxicity have made these nanomaterials superior to other conventional fluorescent materials such as inorganic QDs and organic dyes.…”