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Gene silencing using small interfering RNAs (siRNAs) is a selective and promising approach for treatment of numerous diseases. However, broad applications of siRNAs are compromised by their low stability in a biological environment and limited ability to penetrate cells. Nanodiamonds (NDs) coated with cationic polymers can enable cellular delivery of siRNAs. Recently, we developed a new type of ND coating based on a random copolymer consisting of (2-dimethylaminoethyl) methacrylate (DMAEMA) and N -(2-hydroxypropyl) methacrylamide (HPMA) monomers. These hybrid ND-polymer particles (Cop + -FND) provide near-infrared fluorescence, form stable complexes with siRNA in serum, show low toxicity, and effectively deliver siRNA into cells in vitro and in vivo . Here, we present data on the mechanism of cellular uptake and cell trafficking of Cop + -FND : siRNA complexes and their ability to selectively suppress mRNA levels, as well as their cytotoxicity, viability and colloidal stability. We identified clathrin-mediated endocytosis as the predominant entry mechanism for Cop + -FND : siRNA into U-2 OS human bone osteosarcoma cells, with a substantial fraction of Cop + -FND : siRNA following the lysosome pathway. Cop + -FND : siRNA potently inhibited the target GAPDH gene with negligible toxicity and sufficient colloidal stability. Based on our results, we suggest that Cop + -FND : siRNA can serve as a suitable in vivo delivery system for siRNA.

Photoluminescent gold nanoclusters are widely seen as a promising candidate for applications in biosensing and bioimaging. Although they have many of the required properties, such as biocompatibility and photostability, the luminescence of near infrared emitting gold nanoclusters is still relatively weak compared to the best available fluorophores. This study contributes to the ongoing debate on the possibilities and limitations of improving the performance of gold nanoclusters by combining them with plasmonic nanostructures. We focus on a detailed description of the emission enhancement and compare it with the excitation enhancement obtained in recent works. We prepared a well-defined series of gold nanoclusters attached to gold nanorods whose plasmonic band is tuned to the emission band of gold nanoclusters. In the resultant single-element hybrid nanostructure, the gold nanorods control the luminescence of gold nanoclusters in terms of its spectral position, polarization and lifetime. We identified a range of parameters which determine the mutual interaction of both particles including the inter-particle distance, plasmon-emission spectral overlap, dimension of gold nanorods and even the specific position of gold nanoclusters attached on their surface. We critically assess the practical and theoretical photoluminescence enhancements achievable using the above strategy. Although the emission enhancement was generally low, the observations and methodology presented in this study can provide a valuable insight into the plasmonic enhancement in general and into the photophysics of gold nanoclusters. We believe that our approach can be largely generalized for other relevant studies on plasmon enhanced luminescence.

Photoluminescent (PL) gold nanoclusters (AuNCs) show many advantages over conventional semiconductor quantum dots, however, their application potential is limited by their relatively low absorption cross-section and quantum yield. Plasmonic enhancement is a common strategy for improving the performance of weak fluorophores, yet in the case of AuNCs this method is still poorly explored. Here a robust synthetic approach to a compact plasmonic nanostructure enhancing significantly the PL of AuNCs is presented. Two gold nanostructures, AuNCs and plasmonic gold nanorods (AuNRs), are assembled in a compact core-shell nanostructure with tunable geometry and optical properties. The unprecedented degree of control over the structural parameters of the nanostructure allows to study the effects of several parameters, such as excitation wavelength, AuNC-AuNR distance, and relative loading of AuNCs per single AuNR. Consequently, a more general method to measure and evaluate enhancement independently of the absolute particle concentrations is introduced. The highest PL intensity enhancement is obtained when the excitation wavelength matches the strong longitudinal plasmonic band of the AuNRs and when the separation distance between AuNCs and AuNRs decreases to 5 nm. The results presented are relevant for the application of AuNCs in optoelectronic devices and bioimaging.

Fluorophores functionalized with heavy elements show enhanced intersystem crossing due to increased spin-orbit coupling, which in turn shortens the fluorescence decay lifetime (τPL). This phenomenon is known as the heavy-atom effect (HAE). Here, we report the observation of increased τPL upon functionalisation of near-infrared photoluminescent gold nanoclusters with iodine. The heavy atom-mediated increase in τPL is in striking contrast with the HAE and referred to as inverse HAE. Femtosecond and nanosecond transient absorption spectroscopy revealed overcompensation of a slight decrease in lifetime of the transition associated with the Au core (ps) by a large increase in the long-lived triplet state lifetime associated with the Au shell, which contributed to the observed inverse HAE. This unique observation of inverse HAE in gold nanoclusters provides the means to enhance the triplet excited state lifetime.

Diamond nanoparticles (nanodiamonds) can transport active drugs in cultured cells as well as in vivo. However, in the latter case, methods allowing the determination of their bioavailability accurately are still lacking. A nanodiamond can be made fluorescent with a perfectly stable emission and a lifetime ten times longer than that of tissue autofluorescence. Taking advantage of these properties, we present an automated quantification method of fluorescent nanodiamonds (FND) in histological sections of mouse organs and tumors, after systemic injection. We use a home-made time-delayed fluorescence microscope comprising a custom pulsed laser source synchronized on the master clock of a gated intensified array detector. This setup allows ultra-high-resolution images (120 Mpixels in size) of whole mouse organ sections to be obtained, with subcellular resolution and single-particle sensitivity. As a proof-of-principle experiment, we quantified the biodistribution and aggregation state of new cationic FNDs capable of transporting small interfering RNA inhibiting the oncogene responsible for Ewing sarcoma. Image analysis showed a low yield of nanodiamonds in the tumor after intravenous injection. Thus, for the in vivo efficacy assay, we injected the nanomedicine into the tumor. We achieved a 28-fold inhibition of the oncogene. This method can readily be applied to other nanoemitters with ≈100 ns lifetime.

Nano-optodes with a diamond core coated with a double stimuli-responsive polymeric shell reversibly respond to pH and temperature changes. Swelling and collapsing of the shell are accompanied by changes in the charge of the polymer. Changes in the fluorescent spectra of nitrogen-vacancy centers ratiometrically indicate pH and temperature.

The increase in nonradiative pathways with decreasing emission energy reduces the luminescence quantum yield (QY) of near-infrared photoluminescent (NIR PL) metal nanoclusters. Efficient surface ligand chemistry can significantly improve the luminescence QY of NIR PL metal nanoclusters. In contrast to the widely reported but modestly effective thiolate ligand-to-metal core charge transfer, we show that metal-to-ligand charge transfer (MLCT) can be used to greatly enhance the luminescence QY of NIR PL gold nanoclusters (AuNCs). We synthesized water-soluble and colloidally stable NIR PL AuNCs with unprecedentedly high QY (∼25%) upon introduction of triphenylphosphonium moieties into the surface capping layer. By using a combination of spectroscopic and theoretical methods, we provide evidence for gold core-to-ligand charge transfer occurring in AuNCs. We envision that this work can stimulate the development of these unusually bright AuNCs for promising optoelectronic, bioimaging, and other applications.

Core-shell nanoparticles based on fluorescent nanodiamonds coated with a biocompatible N-(2-hydroxypropyl)methacrylamide copolymer shell were developed for background-free near-infrared imaging of cancer cells. The particles showed excellent colloidal stability in buffers and culture media. After conjugation with a cyclic RGD peptide they selectively targeted integrin αvβ3 receptors on glioblastoma cells with high internalization efficacy.

A novel approach for preparation of ultra-bright fluorescent nanodiamonds (fNDs) was developed and the thermal and kinetic optimum of NV center formation was identified. Combined with a new oxidation method, this approach enabled preparation of particles that were roughly one order of magnitude brighter than particles prepared with commonly used procedures.