Effect of Surface Chemistry on the Fluorescence of Detonation

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The Effect of Surface Chemistry on the Fluorescence of Detonation Nanodiamonds Philipp Reineck, Desmond W. M. Lau, Emma R Wilson, Kate Fox, Matthew R. Field, Cholaphan Deeleepojananan, Vadym N. Mochalin, and Brant C. Gibson ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b04647 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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The Effect of Surface Chemistry on the Fluorescence of Detonation Nanodiamonds

Philipp Reineck1, Desmond W.M. Lau1, Emma R. Wilson1, Kate Fox2, Matthew R. Field3, Cholaphan Deeleepojananan4, Vadym N. Mochalin4, 5, Brant C. Gibson1

1

ARC Centre of Excellence for Nanoscale BioPhotonics & School of Science, RMIT University, Melbourne, VIC 3001, Australia. 2

3

School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia

RMIT Microscopy and Microanalysis Facility (RMMF), RMIT University, Melbourne, Victoria 3001, Australia

4

Department of Chemistry, Missouri University of Science & Technology, Rolla, MO 65409, USA 5

Department of Materials Science & Engineering, Missouri University of Science & Technology, Rolla, MO 65409, USA

Abstract

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Detonation nanodiamonds (DNDs) have unique physical and chemical properties that make them invaluable in many applications. However, DNDs are generally assumed to show weak fluorescence, if any, unless chemically modified with organic molecules. We demonstrate that detonation nanodiamonds exhibit significant and excitation wavelength dependent fluorescence from the visible to the near-infrared spectral region above 800 nm - even without the engraftment of organic molecules to their surfaces. We show that this fluorescence depends on the surface functionality of the DND particles. The investigated functionalized DNDs, produced from the same purified DND, are hydrogen, hydroxyl, carboxyl, ethylenediamine, octadecylamine terminated, as well as the as-received poly-functional starting material. All DNDs are investigated in-solution and on a silicon wafer substrate and compared to fluorescent highpressure high-temperature nanodiamonds. The brightest fluorescence is observed from octadecylamine functionalized particles, which is more than 100 times brighter than the least fluorescent particles, carboxylated DNDs. The majority of photons emitted by all particle types likely originates from non-diamond carbon. However, we locally find bright and photostable fluorescence from nitrogen-vacancy centers in diamond in hydrogenated, hydroxylated and carboxylated detonation nanodiamonds. Our results contribute to understanding the effects of surface chemistry on the fluorescence of DNDs and enable the exploration of the fluorescent properties of DNDs for applications in theranostics, as non-toxic fluorescent labels, sensors, nanoscale tracers, and many others where chemically stable and brightly fluorescent nanoparticles with tailorable surface chemistry are needed.

Keywords: Detonation nanodiamonds, fluorescence, functionalization, nitrogen-vacancy center, diamond, sp2 carbon, carbon dots


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Detonation nanodiamond (DND) is a unique material: it consists of diamond particles of about 5 nm in size, can be produced in commercial quantities and offers many exceptional mechanical, chemical, electronic and optical properties of bulk diamond at the nanoscale.1 DND has a wide range of applications from tribology2 to polymer- and metal-matrix composites3,4 to theranostics.5,6 While many physical and chemical properties of detonation nanodiamod have been studied extensively,1,7 its fluorescence is still poorly understood although it has been investigated in several publications.8–17 This is in stark contrast to generally larger (> 10 nm) milled high-pressure high-temperature (HPHT) nanodiamonds, which can be processed to contain high concentrations (> 10 ppm) of the most widely known fluorescent defect in diamond - the nitrogen-vacancy (NV) color center. In general, DNDs are believed to be mostly nonfluorescent unless modified with organic molecules10,18 or fragments of sp2 carbon and only weak NV emission was observed in a small fraction of purified and deaggregated DNDs.8 However, studies showed that NV fluorescence in DND can be strongly enhanced via sintering,19 NV PL was studied in irradiated and annealed material11,20 and can even be found in mostly unprocessed DNDs.9,14 However, the proximity of NV centers to the diamond surface has been shown to strongly inhibit NV emission in diamond particles of less than 10 nm.21 In general, carbon can give rise to fluorescence through three mechanisms: 1. Delocalized π electrons in aromatic hydrocarbons. This is the cause for strong absorption and re-emission of photons in organic fluorophores, which remain among the brightest fluorescent materials known.22 It has been shown that the stacking of simple aromatic molecules like anthracene or pyrene into nanoparticulate structures also reproduces many of the optical properties of so-called carbon dots,23 which generally consist of graphitic (sp2) or amorphous carbon. Conjugated carbon systems in isolated sp2 hybridized islands have also been used to explain the fluorescence


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properties of single layered graphene oxide.24 2. Defects in sp2 carbon. Graphene is a semi-metal without an optical band gap. When treated with an oxygen plasma it becomes fluorescent25,26 and shows emission properties similar to those observed for graphene oxide produced via wet chemical methods.27 Oxygen, nitrogen and other surface defects in sp2 carbon have been suggested to be the main cause for the bright and excitation wavelength dependent fluorescence of carbon dots.28 Surface defects in sp2 carbon-based nanoparticles are routinely used to ‘explain’ the fluorescence properties of carbon dots. However, the underlying photo-physics are not yet understood. 3. Optically active crystal defects in diamond. A large number of such defects have been identified.29 Most of these have been investigated in far less detail compared to the NV center, mainly because their creation is challenging and large scale fabrication not feasible in many cases.29 Here, we investigate the fluorescence properties of hydrogen-, hydroxyl-, carboxyl-, ethylenediamine-, octadecylamine- functionalized DNDs as well as the as-received polyfunctional starting material. The moieties attached to the DND surface, as well as hydrogen are known to be non-fluorescent on their own and are therefore well-suited to study the surface chemistry effects on the intrinsic DND fluorescence. We demonstrate that the fluorescence properties strongly depend on the surface chemistry of the DND particles. All particle types are investigated in-solution and in a dry state on a silicon wafer substrate using fluorescence spectroscopy and confocal microscopy. Results are compared to ca. 100 nm-sized HPHT nanodiamonds containing high concentrations of nitrogen-vacancy centers. Most DND particle types show strongly excitation wavelength dependent fluorescence from the visible to the nearinfrared (NIR) spectral region. Our results suggest that most of the observed fluorescence originates from residual non-diamond carbon. However, we locally also find significant


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fluorescence form NV centers in hydrogenated, hydroxylated and carboxylated DNDs. Our results highlight the need for a better understanding of the photophysics of fluorescence originating from non-diamond carbon and the effects of non-diamond carbon and impurities therein on defect-based diamond core fluorescence. Results and discussion Materials characterization Commercially available DND particles were used as-received (UD90 grade, NanoBlox Inc., USA) and functionalized according to reported protocols to obtain hydrogenated (DND-H),30 hydroxylated (DND-OH),31 carboxylated (DND-COOH)32 as well as ethylenediamine (DNDEDA)33 and octadecylamine (DND-ODA)18 functionalized particles. The as-received particles are referred to as DND throughout the manuscript. The particles were characterized using Fourier transform infrared spectroscopy (FTIR), UV-visible absorption spectroscopy, dynamic light scattering (DLS) and zeta potential measurements, and Raman spectroscopy. A summary of their basic properties is shown in Figure 1. All particles were dispersed in water except DNDODA, which is strongly hydrophobic and was dispersed in chloroform. The primary particle size of DNDs is 4 - 6 nm. In dispersion all particles were present in aggregates of 40 nm or larger in size and showed polydisperse size distributions (Figure 2A). All particles except DND-EDA showed good colloidal stability after the removal of larger aggregates via centrifugation (1000 rcf for 5 min, see Methods section for details). DND-EDA particles were not stable in solution and sedimented within less than an hour, most likely due to the reduction of the initial highly negative zeta potential of DND-COOH during the wet chemical attachment of ethylene diamine (EDA) to produce DND-EDA.33 The surfaces of all other particles dispersed in water carry strong positive (DND-H) or negative charges (DND-OH, DND-COOH, DND), while DND-EDA


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is not as strongly charged, which is consistent with the particles’ colloidal instability. DND-ODA is only weakly charged and sterically stabilized in chloroform. Despite the visibly different colors of the DND powders, the extinction spectra of all particles are dominated only by scattering from small particles. See Supporting Information (SI) Figure S1 for extinction spectra of all samples. It is important to note that in this study DND-COOH was a precursor for all other chemically modified nanodiamonds.

Figure 1. Overview of the as-received DND and functionalized detonation nanodiamond materials produced from DND-COOH. ‘Diameter’ refers to the diameter of the particles in dispersion as determined via dynamic light scattering. As-received DNDs have graphitic (sp2 carbon) or amorphous carbon shells with highly polyfunctional surfaces exhibiting carboxylic acid, ester, ether, and ketone groups1,7,32 among others. Many chemical and physical processes are known to remove non-diamond carbon including strong acid treatments, annealing in air1,7,32 and ozone treatments.34 However, even with highly efficient processes, up to several weight percent of non-diamond carbon generally remains in detonation nanodiamond samples.32 In this study the sp2 non-diamond and sp3 diamond content of all samples was estimated based on electron energy loss spectroscopy (EELS). The results are summarized in Table 1. Details on the derivation of the shown values from EELS spectra as well as TEM images can be found in Figures S2 - S6 in the SI. DND-COOH has the highest diamond content of about 96 wt% while all other samples contain significantly more sp2 bonded carbon (between 19 wt% and 30 wt%). ODA and EDA both contain sp3 hybridized non-diamond carbon in the form of hydrocarbon chains. However, the EELS signal of the hydrocarbon chains couldn’t be separated from the sp2 carbon peak in our samples (see SI for details). Therefore, we


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note that the EELS-derived values for sp2 bonded carbon quoted for DND-ODA and DND-EDA throughout the manuscript include contributions from sp3 hydrocarbon chains. Table 1. Estimates of the fraction of sp3 diamond and non-diamond carbon present in the different samples based on EELS. sp3 diamond (wt%)

sp2 and nondiamond sp3 carbon (wt%)

DND

81

19

DND-ODA

74

26

DND-EDA

83

17

DND-H

70

30

DND-OH

76

24

DND-COOH

94

6

Sample

The presence of the various surface functionalities was verified using Fourier-transform infrared spectroscopy (FTIR) as shown in Figure 2B. DND-ODA exhibits peaks at 1650 cm-1 and between 2800 cm-1 and 3000 cm-1. These correspond to the presence of amides and C-H stretch vibrations, respectively. The presence of amides is direct evidence of the covalent attachment of ODA to the DND particle, while the C-H peaks are characteristic of long hydrocarbon chains in DND-ODA. The same amide peak is present in the DND-EDA spectrum, again indicating the formation of a covalent bond between EDA and the DND surface. However, the C-H peak is much less pronounced since the EDA molecule only features a two carbon atoms long aliphatic chain between the two end amino groups. A peak at 1630 cm cm-1 is also characteristic of O-H bending, which is clearly present in DND-COOH, but most pronounced in DND-OH. DND-OH, which is produced by reduction of DND-COOH, also has a greatly reduced signal in the region of the C=O stretch, an evidence of C=O conversion into C-OH. The presence


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of a clear peak around 1750 cm-1 indicates the presence of C=O bonds in DND-ODA, DNDEDA and DND-COOH. Its absence in the DND-H spectrum and the appearance of the C-H peaks between 2800 cm-1 and 3000 cm-1 shows the presence of CH2 and CH3 bonds, an evidence of successful hydrogenation of the diamond surface. Peaks indicative of OH groups (bending at 1650 cm-1 and stretching at 3300 cm-1) in the DND-H sample are most likely due to water adsorbed on the surface and covalently bonded OH groups that could not be converted into C-H in these conditions.30 We also investigated the samples using Raman spectroscopy. All particles except DND-ODA show a clear sp3 diamond peak around 1330 cm-1 as well as a broader peak around 1620 cm-1. All particle spectra exhibit a more or less pronounced upward slope towards longer wavenumbers, which is caused by fluorescence. A peak close to 1620 cm-1 is often attributed to the G-band of graphite-like sp2 carbon. However, in the presence of surface functional groups, the analysis of the sp2 contribution to the overall Raman signal is challenging since various oxygen containing functional groups (e.g. O-H) can significantly contribute in this spectral region.35 No diamond sp3 carbon peak is present in the Raman spectrum of the DND-ODA sample. However characteristic diamond lattice features can be clearly seen in high-resolution TEM images (see SI Figure S6 for TEM images of all samples) and 74% sp3 diamond carbon content was estimated from EELS experiments in DND-ODA (see Table 1).

Figure 2. A: Particle size distribution of DND particles dispersed in water (all except DNDODA) and chloroform (DND-ODA) determined by dynamic light scattering. B: FTIR spectra of the DND powders recorded in KBr pellets. C: Raman spectra of particles on a silicon substrate obtained using 532 nm laser excitation.


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In-solution spectroscopy Normalized fluorescence spectra of all particles in solution are shown in Figure 3 for excitation wavelengths from 400 nm to 700 nm. Spectra for longer excitation wavelengths are omitted in case of DND-EDA, DND-OH and DND-COOH due to very low fluorescence signals. For all samples except DND-ODA, the Raman signal of the solvent was on the same order of magnitude and, in the case of DND-OH and DND-COOH, significantly stronger than the fluorescence of the particles. Therefore, the Raman spectrum of the solvent was subtracted from all spectra, which in particular in the case of DND-OH and DND-COOH causes slight distortions of the resulting curves. See SI Figure S9 for the raw spectra and details on the data acquisition and analysis.

Figure 3. Normalized fluorescence spectra of all particles in solution. Particles were excited at different wavelengths between 400 nm and 700 nm as indicated in each graph. Raman spectra of the solvent have been subtracted from all spectra (see main text and SI for more details). For DND, DND-ODA and DND-EDA, the spectral position of the emission peak (λem) increases approximately linearly with the excitation wavelength (λex) between 400 nm and 600 nm (Figure 4A). At longer excitation wavelengths the emission peak position cannot be clearly identified in any sample due to a decrease in the Stokes shift with increasing λex. DND-H, on the other hand, shows only a very weak increase in λem with increasing excitation wavelength. Due to a very weak overall fluorescence signal, a clear excitation wavelength dependence cannot be reliably inferred from the spectra of DND-OH and DND-COOH. However, both types of nanodiamond also show a trend towards longer λem with increasing λex. Figure 4A also shows


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λem as a function of λex for HPHT nanodiamonds with a high concentration of NV centers. Here, λem also shifts to shorter wavelengths with decreasing λex. This is caused by a more pronounced conversion of the NV- charge-state to the neutral NV0 charge-state with increasing photon energy.9 See SI Figure S11 for HPHT ND fluorescence spectra. All DND particles are most efficiently excited at λex = 450 nm, except DND-EDA, which shows a continued increase in fluorescence brightness with decreasing λex down to 400 nm (the shortest λex in our experiments, Figure 4B). The fluorescence brightness strongly decreases for all particles towards longer excitation wavelengths. HPHT ND particles are most efficiently excited at 550 nm, which is close to the absorption maximum of the NV center in diamond.36 The fluorescence brightness of the particles varies by more than two orders of magnitude between the brightest DND-ODA and the very weakly fluorescing DND-COOH. DND-OH is about two times brighter than DND-COOH, while DND, DND-H and DND-EDA are all of the same brightness upon 450 nm excitation and more than ten times brighter than DND-COOH.

Figure 4. A: Fluorescence peak position λem as a function of excitation wavelength λex. B: Relative fluorescence brightness of DND and HPHT particles in solution for the different excitation wavelengths λex. For the brightness comparison, all spectra were corrected for differences in NP concentration and excitation intensities at different λex. C: Time-resolved fluorescence decay traces of DND-ODA, HPHT ND and the instrument response function (IRF) with and without a long-pass filter. Dashed lines represent single exponential fits to the data and the decay time constants are given in the graph.


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Time-resolved fluorescence decay traces of all particles in solution were acquired and the decay curves of DND-ODA and HPHT ND are shown in Figure 4C together with two instrument response functions (IRF). Due to the strong scattering of all particles in solution and weak fluorescence intensity of some samples, spectral filtering was required. The introduction of a long-pass filter leads to a significant broadening of the IRF. All nanodiamond types except DND-ODA showed a relatively fast dominant fluorescence lifetime of less than 1 ns that could not be deconvoluted from the IRF. See SI Figure S12 for decay traces of all particles. The longer components of the decay for DND-ODA and HPHT ND were estimated using a single exponential fit as indicated by the dashed lines in Figure 4C to be 5 ns and 13 ns. The number of publications investigating the fluorescence of DND particles in solution is very limited to date.17,18,37–39 It must be stressed that the properties of detonation nanodiamonds strongly vary between different providers of the ‘raw’ material12 and that small variations in the processing protocols used for purification and functionalization can have significant effects on the particles’ physical and chemical properties and their dispersion behavior. Hence, one must be cautious when comparing the properties of materials produced from different raw materials and following different protocols. To avoid this, we have used one batch of DND to produce DNDCOOH and all other chemically modified DNDs were produced from this DND-COOH. The synthesis and bright fluorescence of DND-ODA was first reported by Mochalin et. al.18 However, only blue fluorescence has been demonstrated and neither the strong shift of λem as a function λex observed here is reported, nor the fluorescence lifetime. Vervald et. al. show fluorescence spectra of DND-COOH using 405 nm excitation, which are slightly blue-shifted compared to the spectrum shown in Figure 3F.17 In another recent study, poly-functional and fluorescent particles have been produced from DND particles in water via laser ablation.37 An


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excitation wavelength dependence of the emission spectra was also observed (only for λex = 360 nm - 500 nm) and rationalized with different ‘surface states’ associated with the various functional groups present on the particle surfaces. Dolenko and coworkers have investigated the fluorescence of fluorinated, hydroxylated and carboxylated DND particles in water (λex= 488 nm) and report weak fluorescence (relative to the water Raman signal) for all functionalizations.16 Two other reports mostly focus on using fluorescence from DND NPs in solution-based applications.38,39 To the best of our knowledge the fluorescence of DND, DNDEDA, DND-H and DND-OH synthesized from the same purified and well-characterized nanodiamond, have not been investigated in solution to date. Confocal fluorescence imaging and spectroscopy The optical properties of all samples were also analyzed on a silicon wafer substrate using a custom-built scanning confocal fluorescence microscope (see Methods section for details). 40 µL of nanoparticle solution was drop-cast onto a silicon wafer at a temperature of 100 °C on a hotplate and allowed to dry. All imaging and spectroscopy was carried out using 500 nm pulsed laser excitation at 5 MHz repetition rate and fluorescence was collected at wavelengths > 540 nm. Confocal fluorescence images of all samples are shown in Figure 5 (note the different intensity scales in the images). Due to the surface properties of the differently functionalized particles, the morphology and thickness of the resulting dry particle layers is significantly different, making a quantitative assessment of the fluorescence brightness challenging. However, it is worth noting that the average fluorescence intensity in the confocal images shown in Figure 5 is in good qualitative agreement with the in-solution results for the fluorescence brightness: DND-ODA exhibits the strongest fluorescence, almost two orders of magnitude stronger than DND-COOH. DND-OH is about twice as bright as DND-COOH while all other particles are


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between these extremes. See SI Figure S14 for the relative fluorescence brightness of all particles deposited on a silicon wafer substrate. DND, DND-H, DND-OH, and DND-COOH form relatively smooth layers with spots of higher brightness, which are from 300 nm (diffraction limited and possibly smaller) to several micrometers in size (also see bright-field images in the SI Figure S13). Large aggregates of DND-EDA particles > 1 µm are found in agreement with the large agglomerate sizes obtained from DLS measurements. Both DND-ODA and DND-H particles also strongly cluster during the drying process and form highly fluorescent regions.

Figure 5. Confocal fluorescence images of all particle types drop-cast onto a silicon wafer. All images were acquired under identical imaging conditions (λex= 500 nm, 100 µW, fluorescence collected above 540 nm). Note that the intensity scales differ between images. Only 10% of photons were used for imaging and 90% for spectroscopy. Hence, ‘counts per second’ (CPS) values in this figure represent CPS values detected by the avalanche photodiode multiplied by 10. Circles in panels E and F indicate locations where NV fluorescence was observed. The confocal fluorescence images in Figure 5 were analyzed in two ways: 1. by analyzing individual locations of interest. 2. via averages obtained while scanning part of an image. Fluorescence spectra and time-resolved fluorescence decay traces obtained in both ways are shown in Figure 6. The individual locations investigated were bright spots that can easily be identified in Figure 5 E and F (examples are highlighted by circles). In all other images these spots are difficult to identify. In the DND sample they are very rarely found, while in Figure 5 B-


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D the large number of brightly fluorescing spots and the high overall fluorescence makes their identification difficult. To obtain results averaged over many particles, regions of homogenous fluorescence were imaged (10 x10 µm, λex=500 nm) with spectra and fluorescence decays recorded while scanning. These spectra and decay traces are shown in Figure 6 A and B, respectively. The spectra of all particle types exhibit a similar full-width-half-maximum of about 150 nm. The Stokes shift varies and is smallest for DND-ODA and DND-EDA, and largest for DND-COOH and DND particles. All emission spectra are blue-shifted with respect to a typical NV emission spectrum from HPHT NDs. A direct comparison of the fluorescence spectra obtained in dispersion vs. on a silicon substrate is shown in the SI Figure S15. All particle types show a multi-exponential fluorescence decay (Figure 6B). This implies the presence of more than one fluorescence decay pathway for optically excited states. Using deconvolution with the instrument response function (IRF) we find that all traces are well approximated by bi-exponential decays. The fluorescence lifetimes obtained from this fitting process are shown in Table 2. DND showed a decay component well below 1 ns, which was too short to resolve. All other particle decay traces are dominated by a short lifetime component of between 0.8 ns (DND-EDA) and 1.4 ns (DND-ODA). DND-ODA has a significant contribution from a longer-lived state with τ2 = 5.8 ns, which is close to the value obtained for the particles in solution. All other particles (except DND) show comparable τ2 values between 3.9 ns and 6.5 ns, but with a significantly lower amplitude. The HPHT ND particles also show a fast (9.1 ns) and slow (44 ns) lifetime component, which contribute almost equally to the overall decay with amplitudes of 44% and 56% respectively.


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Figure 6. Fluorescence spectra (A,C) and time-resolved fluorescence decay traces (B,D) of different DNDs drop-cast onto a silicon wafer. A typical NV fluorescence spectrum of HPHT NDs is shown in panel A and C for comparison. A, B: Average fluorescence spectra and decay traces observed for most particles on the substrate. C,D: Fluorescence spectra and decay traces of individual bright spots that show typical characteristics of NV fluorescence. Note the different xaxis scaling in panel B and D. λex = 500 nm at 100 µW excitation intensity in all cases. Table 2. Average fluorescence lifetimes and relative amplitudes for all samples drop-cast on a silicon substrate.

τ1 [ns]

DND

DND-H

DNDOH

DNDCOOH

DNDEDA

DNDODA

HPHT NDs

10 ns. For DND-H only 1 and DNDOH 3 out of 20 showed these characteristics. Broad fluorescence spectra from DND particles on glass or Si substrates upon excitation with green or blue light, similar to the ones in Figure 6A have been reported.8,9,11,13,14,20,40 Several studies use proton or electron irradiated and vacuum annealed DND particles to increase the number of NV defects in the material after chemical or physical purification.11,20,40 Other studies investigate as-received9 or chemically purified8,14 material without prior irradiation. Smith and colleagues11 attribute a fast fluorescence decay, similar to the one seen in Figure 6B for DND, to graphitic carbon. A longer lifetime component > 10 ns was attributed to NV fluorescence in time-gated spectroscopy experiments. The spectra show a featureless emission from sp2 carbon in the first few nanoseconds after excitation. Nitrogen-vacancy zero-phonon lines appear in the spectrum when recorded with a 20 ns delay. However, Vlasov et. al. suggested that the clear NV fluorescence originates from larger diamond crystals in DND of > 30 nm and not from individual or clustered 3-6 nm sized DND particles.40 Kirmani et. al. further illuminate the chemical structure of non-diamond carbon around the diamond core and its role in the quenching of coreoriginating fluorescence.14 Clear NV signatures like the ones shown in Figure 6 C and D have also been reported in DND particles. However, most of these studies used irradiated and annealed DND material.11,20,40 Our group has previously reported NV fluorescence of comparable brightness from as-received DND material (from a different provider than the material used in the present study). The photostability of the DND particles was investigated by positioning the beam in an area of average fluorescence brightness and recording the collected fluorescence over time. The resulting photobleaching traces, normalized at time t = 0, are shown in Figure 7. In general, all


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DND particle types show photobleaching behavior. However, the individual spots showing NV fluorescence characteristics (Figure 6 C and D) were found to be highly photostable - in agreement with our previous report9 (not shown here). The fluorescence of DND-ODA decreases by more than 75% within less than 30 s. DND, DND-EDA, DND-H, and DND-OH show better photostability characteristics, while DND-COOH shows the highest photostability with a reduction in fluorescence of about 20% during the first 5 minutes of illumination. All photostability traces were well fitted by a single (DND-COOH) or bi-exponential decay (all others). The characteristic photobleaching times for all DND particles are summarized in Table 3. It is worth noting that the photostability is approximately anti-correlated with the fluorescence brightness shown in Figure 4, i.e. the brighter a particle is, the less photostable it is - with the exception of HPHT ND, which exhibits perfect photostability (not shown here). All traces were background-corrected. Hence, background effects can be excluded as an explanation for the observed differences in photobleaching behavior. No changes in fluorescence spectra obtained before and after photobleaching were found for any sample. The excitation intensity of 100 µW used here (λex = 500 nm) is at least one order of magnitude lower than the excitation intensities used in Raman experiments performed on the same samples at a similar wavelength (532 nm). Raman spectra were found to remain unchanged upon repeated acquisition. The illuminated areas were also visually inspected for any laser light induced changes before and after the Raman experiments and no changes were found. This suggests that the light intensities used in the photostability experiments are also unlikely to induce significant changes to the DND particles such as graphitization. However, it is likely that the employed light intensity of about 1.4 × 105 W cm-2 in the focal spot is high enough to oxidize functional surface groups and molecules of illuminated particles, which may cause the observed photobleaching.


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Figure 7. Photostability of DND particles: fluorescence intensity as a function of time. All traces were normalized to the intensity at t=0 s to show the different bleaching dynamics. Table 3. Bleaching time constants obtained from an exponential (COOH) or bi-exponential (for all other samples) fit to the data as shown in Figure 7. τB1 [s]

τB2 [s]

DND-COOH

121

-

DND-OH

15

144

DND-H

13

142

DND-EDA

10

104

DND

8.3

117

DND-ODA

3.5

102

Origins of fluorescence We discuss two distinct types of fluorescence that were found to originate from our detonation nanodiamond samples: A. Carbon dot-like fluorescence from graphitic or amorphous carbon and B. fluorescence from NV centers in diamond. 1. Carbon dot-like fluorescence The particles investigated here are clusters > 40 nm of individual 4-6 nm sized DND particles (see Figure 8). The amount of non-diamond carbon present on the surface or around these clusters strongly depends on the processing and functionalization of the particles. We find the particles with the highest sp3 diamond content (DND-COOH) to show the weakest overall fluorescence both in-solution and on a silicon substrate. The non-diamond carbon content in all


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other particles varies maximally by about 11%. Yet, their fluorescence properties vary dramatically, which emphasizes the importance of the surface functionalization. DND-ODA, EDA, -OH, and -H are all created from DND-COOH, i.e. from a very weakly fluorescing material consisting of about 96% diamond. During the various functionalization processes the chemical structure of only the outermost carbon layers is changed and results in an increase of non-diamond carbon. This graphitic (sp2) and amorphous carbon causes the relatively fast decaying (fluorescence lifetime) and quickly photobleaching fluorescence observed from DNDODA and DND-EDA. This fluorescence and its strong λex dependence is similar to the asreceived polyfunctional and sp2 containing DND material and typical for carbon dots.28 Octadecylamine41 and ethylenediamine42,43 have both been used in literature before for the synthesis of carbon dots. Nitrogen impurities have been identified as one of the causes of carbon dot fluorescence.44 In this sense DND-H stands out as it is also dominated by short-lived fluorescence decay and of comparable brightness to DND-EDA, but lacks the λex dependence and shows a significant contribution from a longer-lived emission above 10 ns (see Figure 6B). Also, the surface chemistry of DND-H is much simpler than that of DND-ODA/EDA and does not involve nitrogen-rich molecules. DND-OH shows a slightly stronger λex dependence, but also significantly weaker overall fluorescence with a short fluorescence lifetime, which most likely also originates form non-diamond carbon. The significant NIR fluorescence of DND, DND-ODA and DND-H in a spectral region above 800 nm is noteworthy (Figure 2 A,B,D). Particularly since only a handful of carbon dots fluorescing in the NIR spectral region have been reported to date.44 However, we also find the fluorescence of DND, DND-ODA and DND-H to be most pronounced in the visible light range. 2. Nitrogen-vacancy fluorescence


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NV fluorescence from DND-H, DND-OH and DND-COOH particles deposited on a substrate can clearly be identified in individual locations. These individual, generally diffraction limited spots are most frequently encountered in the DND-COOH sample. However, the main reason for this could be the low background of the majority of the DND-COOH, which only weakly fluoresces and facilitates the identification. Therefore, occurrence alone is insufficient to draw conclusions regarding the effect of the particle’s surface functionalization on the fluorescence of the embedded NV defects. In the DND-COOH sample we find significant variations in the optical properties of these spots. Their brightness varies by an order of magnitude and we find spectra that are strongly dominated by NV- emission, as well as the ones dominated by NV0 emission (See SI Figure S16) under identical imaging conditions. The slow decay fluorescence components of the NV spots show slightly less variation between 24 ns and 34 ns, but the relative contribution of fast and slow decay also differs by an order of magnitude (see SI Figure S16). These large variations within a single sample necessitate the analysis of a significant number of particles to identify systematic differences between different particle types. Identifying the effect of the surface groups on NV emission and understanding why only a small subset of particle aggregates shows significant NV fluorescence will be the focus of future investigations. The starting material for all particles studied in this work is the same. None of the processing steps can lead to a removal or creation of NV centers from the diamond particles unless the structure of the diamond crystal lattice itself is compromised. Since we know that all DND particles consist of ≥70 % diamond, NV centers are highly likely to be present in all particle types. However, although many of them can be located in solid state experiments, most of these centers are optically inactive or cannot be identified amidst strong carbon dot-like fluorescence.


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Figure 8. Generalized schematic illustration of the structure of the DND particle aggregates, the location of surface modifications and color centers as well as the different sources of fluorescence. Conclusion We have systematically characterized the fluorescence of 5 differently functionalized detonation nanodiamond particles produced from the same carefully purified and characterized starting material, as well as the as-received material in solution and on a silicon wafer substrate. We find a strong excitation wavelength dependence and significant fluorescence ranging from the visible to the NIR spectral region from DND, DND-ODA and DND-EDA particles in solution. Of the simple surface terminations (H, OH, COOH) we find DND-H to show the brightest fluorescence, which only weakly depends on the excitation wavelength. DND-OH and DND-COOH only show weak fluorescence. Based on the characteristic decay time we attribute the fluorescence observed in solution to residual non-diamond carbon present in all samples. Fluorescence from NV centers in diamond is found in DND-H, OH and COOH samples. A summary of the key findings is presented in Figure 9. Pure diamond particles are optically transparent and completely non-fluorescent. The introduction of defects and impurities is known to create fluorescent color centers like the NV center. Our results suggest that the surface chemistry of detonation nanodiamonds is critical for their fluorescence properties in two ways: firstly, through the creation of fluorescent defects on or close to the particle surface via the local reconfiguration of carbon atoms as a result of the functionalization process; secondly, through the interaction of the modified particle surface with


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the optical defects and color centers inside the particles’ diamond core. The fundamental photophysics of both effects are currently poorly understood and we expect our results to spark further research into the fluorescence of DND. Furthermore, our results may enable fluorescence-based imaging or detection of structures and materials in applications such as nanomedicine or materials technology, where DND was thus far only used for its superior physical (mainly mechanical) and chemical properties.

Figure 9. A summary of the key findings of this study. Methods Sample preparation H, COOH, OH and EDA functionalized DND powders were dissolved in deionized water at a concentration of 1 mg/mL and sonicated for 1 h at 150 W using a horn sonicator with a 66% duty cycle. DND-H, DND-COOH, DND-OH and DND-ODA solutions were centrifuged at 1000 rcf for 5 minutes and the supernatant was used for experiments. DND-ODA is well dispersible in chloroform and the centrifugation did not change the concentration significantly. The DNDODA and DND-EDA solutions were diluted to a concentration of 0.2 mg / mL and 0.25 mg/mL in chloroform and water, respectively. The concentrations of all other particles in solution were estimated using nanoparticle tracking analysis (NanoSight NS300, Malvern Instruments) to be 0.4 ± 0.2 mg / mL (DND, DND-COOH and DND-OH) and 0.2 ± 0.1mg / mL (DND-H). These solutions

were

used

for

in-solution

spectroscopy

experiments.

For

confocal

microscopy/spectroscopy experiments 40 µL of NP solution was drop-cast on a silicon wafer


22

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substrate on a hotplate at 100 °C and allowed to dry. HPHT ND particles were purchased from Nabond Technologies and processed as described in the SI. In solution spectroscopy A collimated laser beam (Fianium, WhiteLaseSC 400) was weakly focused into a cuvette with an optical path of 10 mm, 2 mm diameter (FireflySci, Inc., 18FL Micro). The fluorescence was collected, fiber coupled and analyzed with a spectrometer (Princeton Instruments, SpectraPro with a PIXIS CCD camera) to obtain fluorescence spectra. For time-resolved direct fluorescence decay traces, photons were collected with avalanche photo diodes (APD, Excelitas, SPCMAQRH-14) and analyzed with a correlator card (Picoquant, TimeHarp 260). See SI for a schematic drawing of the setup. Confocal imaging and spectroscopy The same light source and detectors used for in-solution spectroscopy were used in a custombuilt confocal fluorescence microscope setup. The excitation beam (λex = 500 nm) was focused onto the sample using a 100x air objective (0.9 NA). Fluorescence was filtered by a 532 nm dichroic and long-pass filter (Semrock) and fiber coupled for detection by the above APD and spectrometer. ASSOCIATED CONTENT Supporting Information. Extinction spectra, EELS spectra and analysis, TEM images, raw insolution fluorescence spectra, schematic illustration of the fluorescence spectroscopy setup, fluorescence spectra of fluorescent HPHT nanodiamonds in water, fluorescence decay traces of all DND types in solution, bright-field microscopy images, brightness comparison of DND particles on Si wafer, comparison of fluorescence spectra of particles in solution and particles on a Si wafer.


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Corresponding Authors * [email protected] * [email protected] Author Contributions VNM, PR, and BCG conceived and planned the study. VNM and CD purified, modified, and characterized the detonation nanodiamond samples using FTIR. PR has performed all optical characterizations. PR has conducted all data analysis (except for EELS), generated all graphs and created all illustrations. DWML has performed all TEM and EELS experiments and data analysis. ERW has performed DLS and zeta-potential measurements. KF and MRF have contributed to the TEM and EELS data analysis and interpretation. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledegements This work has been supported by the ARC Centre of Excellence for Nanoscale BioPhotonics and ARC grants (FT110100225, LE140100131, CE140100003). B. C. G. acknowledges the support of an ARC Future Fellowship. The authors are thankful to Dr. K. Turcheniuk for initial help with synthesis of DND samples. The authors acknowledge the use of the RMIT Microscopy and Microanalysis Facility (RMMF) and the MicroNano Research Facility (MNRF) at RMIT University. References 1.

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DND

DND-H

DND-OH

DND-COOH

DND-EDA

DND-ODA

17

Abbreviation

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OH COOH ketone

H

OH

COOH

Functionalization

diamond

diamond

diamond

diamond

diamond

diamond

Diameter

79 nm

60 nm

50 nm

72 nm

> 300 nm

44 nm

Zetapotential

- 29 mV

+ 56 mV

- 32 mV

- 46 mV

- 19 mV

- 5 mV

water

water

water

Dispersant

water

water

Image Figure 1. Overview of the functionalized detonation nanodiamond materials. ACS Paragon Plus Environment

chloroform

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DND functionalization and material characterization B DND DND-H DND-OH DND-COOH DND-EDA DND-ODA

20

15

Absorption [arb.u.]

Number weighted intensity [%]

A 25

10

C Amide 1 O-H C=O

Diamond

C-H N-H O-H

DND-ODA DND-EDA

DND-COOH

DND-OH DND-H DND-COOH

DND-OH

5

DND

DND-H

0 10

G-band

DND-ODA DND-EDA

Intensity [a.u.]

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2

3

4

5 6 7 8

100

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Particle diameter[nm]

3

4

5

1000

2000

3000 -1

Wavenumber [cm ]

4000

1300

1400

1500

-1

1600

Wavenumber [cm ]

Figure 2. Particle size distribution (A) of the functionalized DND particles dispersed in water (all except DND-ODA) and cholorform (ODA) from dynamic light scattering and FTIR spectra of the DND particle powders (B).


In-solution spectroscopy

Normalized fluorescence

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1.0

DND λex [nm]

A

0.0 1.0

D

600

800

1000 DND-H Wavelength [nm] λex [nm]

DND-EDA λex [nm]

600

800

Wavelength [nm]

1000

C

DND-ODA λex [nm] 400 450 500 550 600 650 700

400 450 500 550 600

E

DND-OH λex [nm]

400 450 500 550 600 650 700

0.5

0.0

B

400 450 500 550 600 650 700

0.5

Data from 7 April

ACS Nano

F

600

800

1000 DND-COOH Wavelength [nm]λex [nm] 400 450 500 550

400 450 500 550

600

800

Wavelength [nm]

1000

600

800

1000

Wavelength [nm]

Figure 3. Normalized excitation and fluorescence spectra of DND and HPHT ND particles in solution. Particles were excited at different wavelengths between 400 nm and 600 nm as indicated in the graphs. Raman spectra of silica particles in water have been subtracted from all spectra. ACS Paragon Plus Environment

In-solution spectroscopy

Page 34 of 40

B

C

700 650 600 HPHT ND DND-COOH DND-H DND-OH DND-ODA DND-EDA DND

550 500 400

450

500

550

600

Excitation wavelength [nm]

HPHT ND DND-COOH DND-H

100

DND-ODA DND-EDA DND-OH DND

10

1

Fluorescence [counts]

Emission peak position [nm]

A

Relative brightness

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

4

10

HPHT ND DND-ODA IRF with LP filter IRF

6 4 2 3

10

13 ns

6 4 2

5 ns

2

10

6 4

0.1

400

500

600

700

2

0

10

Excitation wavelength [nm]

Figure 4. A: Fluorescence peak position as a function of excitation wavelength. B: Relative fluorescence brightness of DND and HPHT particles in solution.

Data from 7 April


20

30

Time [ns]

40

50

Confocal fluorescence microscopy and spectroscopy

Page 35 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

A

DND

B

DND - ODA

C

DND - EDA

D

DND - H

E

DND - OH

F

DND - COOH

Figure 6. Confocal fluorescence images of particles drop-cast onto a silicon wafer. All images were acquired under identical imaging conditions (Ex: 500 nm with 100 µW power, fluorescence collected above 540 nm).ACSNote the maximum fluorescence intensity shown Paragon that Plus Environment in images A-C is an order of magnitude higher than in images D-F.

Confocal fluorescence microscopy and spectroscopy ACS Nano

Averages DND-COOH DND-EDA DND-H DND-ODA DND-OH DND HPHT ND

1.0

0.5

B 106 Fluorescence [counts]


A

5

10

HPHT ND DND-ODA DND-H DND-OH DND-COOH DND-EDA DND IRF

4

10

3

10

12 ns 2

0.0

10 600

700

800

0

20

Wavelength [nm]

40

60

Time [ns]

Individual spots

C

D 1.0

DND-COOH DND-H DND-OH HPHT ND

NV-

0.5 NV0

Fluorescence [counts]


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

4

10

DND-COOH DND-H DND-OH HPHT ND

4 2 3

10

4 2 2

10

4 2

0.0

1

600

700

800

10

0

Wavelength [nm]

50

100

Time [ns] ACS Paragon Plus Environment

150

Page 37 of 40 1.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

ACS Nano

DND - COOH 0.8 DND - OH 0.6 DND - H

0.4

DND - EDA 0.2

DND DND - ODA

0.0 0

ACS Paragon Plus Environment 100 200

Time [s]

300

ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

sp3 carbon sp2 carbon ODA / EDA NV center active NV center non-diamond carbon fluorescence


Page 39 of 40

Sample

Comments

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

•

DND-COOH

• •

DND-OH

DND-EDA

Lowest overall fluorescence Highest diamond content NV Highest occurrence of bright NV emission

•

2nd highest occurrence of bright NV fluorescence

non-diamond carbon / NV

•

Strongest dependence of fluorescence spectra on λex

non-diamond carbon

•

Weakest dependence of fluorescence spectra on λex The only positively charged particle

non-diamond carbon / NV

Shortest fluorescence lifetime Highly poly-functional

non-diamond carbon

DND-H •

•

DND

Fluorescence origins

ACS Nano

•

Longest fluorescence lifetime • Highest overall fluorescence • Lowest photostability •

DND-ODA


sp3

carbon

sp2

carbon

ODA/EDA

NV center

active NV center

non-diamond carbon non-diamond carbon fluor.

ACS Nano

TOC

Detonation nanodiamonds

17

EDA

divided by 10

Excitation: 450 nm

3

Fluorescence Fluorescence

ODA

x10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

polyfunctional H OH

500

600

700

800

Wavelength [nm] Wavelength [nm]


COOH

Effect of Surface Chemistry on the Fluorescence of Detonation

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