Surface Photoluminescence of Oxidized Nanodiamonds: Influence of Environment pH

Abstract

Nanoparticles’ environment, in particular pH, can strongly affect their photoluminescence (PL). This can be especially true for the surface PL of nanodiamonds (NDs) known to be dependent on the composition of surface groups. In this paper, the effect of environmental pH in the range from 2 to 12.5 on the surface photoluminescence of oxidized NDs of various syntheses and surface treatments was investigated. For the first time, the varying changes of the NDs’ PL with the pH were shown, having, however, an essential common feature: they were observed in the same pH regions from 2 to 5 and from 9 to 12.5, while in the pH region from 5 to 9, the PL intensity of all samples was almost constant. Obtained dependences of NDs’ ζ-potentials on pH and quantum mechanical modeling showed that all changes of NDs’ PL are due to the (de)­protonation of surface groups: carboxyl at pH from 2 to 5 and hydroxyl on sp2-hybridized carbon at pH from 9 to 12.5. The varying response of NDs’ PL to such deprotonation shows the possibility of tuning the surface photoluminescence of NDs for use in industry and biomedicine beyond what is possible by only controlling the nanodiamonds’ surface groups.

Dependence of photoluminescence of carbon dots with different surface functionalization on hydrogen factor of water

“…According to the authors of [28], this phenomenon may be a consequence of the fact that the energy of triplet excitation (T 3 ) of the deprotonated form is located slightly below the energy of its singlet state (S 1 ), which makes the speed on interlevel transition from S 2 to T 3 higher in the protonated form than in the deprotonated form. Also, it should be noted that results obtained in this study for the effect of pH on the PL of several types of CD in water are fully compatible with results of similar studies for water suspensions of detonation-synthesized nanodiamonds having large amount of s p 2 -hybridized carbon on their surfaces [29]. In the above-mentioned study confirmed by quantum-chemical methods the hypothesis that the obtained dependence of PL of the s p 2 -hybridized carbon on the environment pH is caused by protonation/deprotonation processes.…”

Quenching of Photoluminescence of Carbon Dots by Metal Cations in Water: Estimation of Contributions of Different Mechanisms

“…It was previously shown that the main reason for the change of photoluminescence of CDs obtained by hydrothermal synthesis with the change of pH of the solution, , similar to other carbon nanoparticles–nanodiamonds, is (de)­protonation of surface groups. Deprotonation of surface groups occurs in the pH range from p K a – 1 to p K a + 1, where p K a is the decimal logarithm of the acid (or alkaline) K a index of certain surface groups.…”

“…This means that small aggregation of nanoparticles occurs in water due to the interaction of richly developed surface groups of various nanoparticles with each other. It should be noted that three types of the main chemically active functional groups on the surface of CDs (carboxyl, hydroxyl and amide) at the specified measured pH value are in different states: carboxyl groups are deprotonated (−COO – ), hydroxyl groups are neutral (−OH), amide groups on sp 2 - and sp 3 -hybridized bases are excessively protonated (−NH 3 + ) and neutral (−NH 2 ), respectively ,− (for more details, see section ). It is most likely that the negative zeta potential of CDs is caused by the quantitative advantage of deprotonated carboxyl groups over positively charged groups.…”

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

“…Therefore, the photoluminescence and applications of fullerenes, CNOs, and NDs are usually limited owing to blue emissions, low QYs, and short Stokes shifts. 303,382 However, oxidation with strong acids; surface passivation by inserting carboxyl, amino, and hydroxyl groups; and covalent surface functionalization by conjugation with fluorescent molecules and specific recognition markers are convenient and effective approaches for improving fluorescent properties endowing biological diagnosing, tracking, and imaging capabilities for fabricating fascinating fullerenes, CNOs, and NDs sensors. 267,322,383,384 Owing to their excellent photoluminescence and multicolor fluorescence emission, which mainly originates from quantum confinement and surface defects, CDs fluorescent properties have attracted widespread interest.…”

“…For 0D CNMs, photoluminescence is the most interesting property, has attracted the most attention, and is how we discovered 0D CNMs for applications. , Similar to pristine fullerenes, pristine CNOs and NDs sometimes exhibit low fluorescence efficiencies and stabilities and poor sensitivities and water solubilities. Therefore, the photoluminescence and applications of fullerenes, CNOs, and NDs are usually limited owing to blue emissions, low QYs, and short Stokes shifts. , However, oxidation with strong acids; surface passivation by inserting carboxyl, amino, and hydroxyl groups; and covalent surface functionalization by conjugation with fluorescent molecules and specific recognition markers are convenient and effective approaches for improving fluorescent properties endowing biological diagnosing, tracking, and imaging capabilities for fabricating fascinating fullerenes, CNOs, and NDs sensors. ,,, …”