Mass production of fluorescent nanodiamonds with a narrow emission intensity distribution
Introduction
Nanodiamond (ND) is a biocompatible carbon nanomaterial that has been recently introduced as a useful platform for construction of nanoprobes and quantum nanosensors for optical [1], [2], [3], [4] and biomedical [5], [6], [7], [8], [9], [10], [11] applications. ND can accommodate lattice point defects, nitrogen-vacancy (NV–) centers, which exhibit an electronic spin resonance between the triplet ground state sublevels that can be detected optically [3], [12]. Because the energies of these sublevels are sensitive to external magnetic fields, the NV centers in ND have prospects for use as a nanoscale magnetic field sensor [13], [14], [15]. These remarkable properties, together with the perfect photostability of NV center fluorescence, the absence of photoblinking for nanocrystals with diameters larger than 5 nm [16], [17], [18], [19] and an emission wavelength in the near-infrared region (low background in a biological environment) [20], have enabled use of NDs in high-impact applications, particularly in the biomedical domain [21], [22], [23], [24], [25], [26], [27]. Fluorescent NDs (fNDs) have been proposed as tools for real-time sensing of voltage-gated ion channels [28] and as chemosensors [29], [30]. They have already been used, for example, as detectors of very low concentrations of paramagnetic ions [31], [32], [33], for tracing of neuronal processes [34], revealing the relations between particle shape and their intracellular fate [35], [36], temperature nanosensing [37], in drug delivery systems [38], [39], [40] and for targeting of various cell types [41].
NV centers are found as rare defects in synthetic diamonds owing to the presence of (i) nitrogen impurities at concentrations of 100–200 ppm and (ii) rare vacancies [42], [43]. However, at nanometer sizes (<20 nm) the fraction of NDs naturally containing at least one NV center is less than 1% [44]. To increase this fraction and/or the concentration of NV centers, one needs to generate more vacancies, which may be done by irradiating the diamond with high-energy particles (i.e., alpha particles [23], [45], [46], protons [47], [48], [49] or electrons [18], [50], [51]). After irradiation, the ND is annealed in an inert atmosphere or in a vacuum [52]. In this process, crystal lattice vacancies created by irradiation thermally recombine with naturally occurring nitrogen impurities [53]. Despite the fact that demand for fNDs is rapidly growing, the available procedures for creating NV centers in large amount (mass of about 1 g) of nanodiamonds per run are limited to solid state techniques, which produce a characteristic non-homogeneous distribution of fluorescence centers in individual ND particles [54].
In this study, we present an approach to fND mass production based on direct irradiation of NDs in aqueous colloidal solution with high-energy (≈16 MeV) protons. We show that fNDs prepared this way contain a larger fraction of fluorescent particles, with a more homogenous distribution of NV centers per particle and fewer lattice damages, compared to NDs irradiated in the pellet state with the same proton beam.
Section snippets
Chemicals
Sodium hydroxide, nitric acid (65%) and sulfuric acid (96%) were purchased from Penta (Czech Republic). Potassium nitrate was purchased from Sigma–Aldrich (Prague, Czech Republic). All chemicals were p.a. quality and were used as received without further purification. Deionized water was prepared with a Millipore Synergy UV Ultrapure water system.
Synthetic NDs containing approximately 100 ppm of natural nitrogen impurities [53] and produced by high pressure high temperature method with size
Results and discussion
The number of vacancies created in the diamond increases as the hadron particles slow down. Most of the damage is therefore caused at the end of the particle path (Bragg peak), contributing to a nonhomogeneous distribution of formed vacancies (and consequently of NV centers). Although for optimal production of NV centers relatively low particle energies (tens of keV) are needed [23], [61], the particles' penetration depth is, in this case, very small and the created higher densities of NV
Conclusions
We showed mass production of fND by irradiating an aqueous suspension of diamond nanocrystals yields a more homogeneous distribution of NV color centers per particle than solid phase pellet irradiation with a three-fold larger fraction of fNDs, which is important for applications such as bioimaging. Moreover, we foresee that lower fluence irradiation in a liquid target followed by high-temperature annealing and air oxidation can also be used to produce fNDs containing exactly one NV center in
Acknowledgments
The work was supported by MZ-VES project Nr. 15–33094A. Irradiations of NDs were carried out at the CANAM infrastructure of the NPI ASCR Rez supported through MŠMT project No. LM2011019. MN, VP and MG would like to acknowledge the financial support from ESF – Grant No. CZ.1.07/2.3.00/20.0306 and CZ.1.05/4.1.00/11.0253. MN would like to acknowledge the project FWO (Flanders) G.0.943.11.N.10. Work of SAZ was supported by a public grant overseen by the French National Research Agency (ANR) as part
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