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Tunable charge transfer on selectively functionalised diamond nanoparticles
Diamond & Related Materials 68 (2016) 78–83
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Tunable charge transfer on selectively functionalised diamond nanoparticles L. Lai a, A.S. Barnard b,⁎ a b
School of Physical Science and Technology, Southwest University, BeiBei District, Chongqing 400715, PR China CSIRO Virtual Nanoscience Laboratory, Parkville, VIC 3052, Australia
a r t i c l e
i n f o
Article history: Received 14 March 2016 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 17 June 2016 Keywords: Nanodiamond Bucky-diamond Ionization potential Electron affinity Charge transfer Simulation
a b s t r a c t For applications in nanomedicine, it has been shown that charge transfer properties are fundamental to the efficient loading and release of therapeutic agents, though it is not always clear whether intrinsic surface reconstructions and chemical conditions will encourage or suppress electron donation or acceptance. In this study we have investigated the relationship between the localised surface charge transfer, the ionization potential and the electron affinity of a model diamond nanoparticle, as a function of the distribution of important intrinsic surface terminations: H, O and OH. The detailed results reveal that diamond nanoparticles can act as electron donors or electron acceptors, depending on the type of surface terminations that are applied, the efficiency of the surface passivation (coverage) and the orientation of the facets where binding occurs. The results suggest that incomplete passivation is not necessarily undesirable, and that by driving charge transfer in different directions (to, or from, the host nanoparticle) by changing the surface chemistry new methods for more tailored applications such as drug delivery may be explored, based on post synthesis processes that do not necessitate modifications to the formation and purification processes already in place. Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.
1. Introduction Undoubtedly one of the most exciting applications for colloidal nanoparticles is in the field of nanomedicine [1–3]. Although there are still many challenges in realising this potential [4,5], there are some cases where research is progressing well toward the clinic [6,7], and others were even at the early stages we are beginning to see how the intrinsic properties of the nanoparticle are playing a crucial role [8,9]. Nanoparticles are useful for imaging [10], detection [11] and drug delivery [12]; provided their surfaces are dressed appropriately [13]. One example in this domain is diamond nanoparticles [14–17], which are already showing great promise for a range of therapies [19–21]. Benefits include increased selectivity [23,24] and sensitivity [22], the ability to more effectively moderate dosage [25] and provide more long term, sustained treatments without burst effusion [26]. It is now possible to produce stable suspensions of nanodiamonds with a hydrodynamic diameter of ~ 3.0 nm, with a narrow size distribution [27], which provides an even greater specific surface area for carrying therapeutic agents, biomarkers and ligands [28,29]. In each case, charge transfer properties are important to interactions of molecules with the surfaces of nanoparticles, including the loading and release of therapeutic agents, and will therefore impact the dose and dose rate during treatment. For example for the positively charged doxorubicin ions (NH3+) ⁎ Corresponding author. E-mail address: [email protected] (A.S. Barnard).
http://dx.doi.org/10.1016/j.diamond.2016.06.007 0925-9635/Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.
to be attracted to diamond nanoparticles the surfaces must be negatively charged, so that one needs to consider variations in the charge equilibrium on the surface sites and the amine group on doxorubicin to control the capture and release, as determined by solvent cations or anions [30]. In short, drugs may be bound to, and released by, the nanoparticle via charge transfer; which is well known in many other drug delivery platforms as well [31–34]. In general, the direction and efficiency of charge transfer depends on whether the underlying diamond nanoparticle acts as a donor or an acceptor, and can be characterized using the sign and value of the ionization potential (the donation of an electron) and the electron affinity (the accepting of an electron). It is possible to make direct measurements, and in the case of the H-terminated diamond surfaces it is well known that the electron affinity is negative [35,36]; though the exact value depends on a variety of factors [37–39]. This includes the type of interface [40,41], and species of surface terminations [43] or atmospheric conditions [42,44]. In addition to this, specific mixtures of surface groups (including H and OH) may result in NEA [45]. Similarly, recent studies have also shown that charge transfer at the nanodiamond surface depends sensitively on the types of particles, surfaces and surrounding medium [46–48]. Given these relationships, it is logical to assume it is possible to use them to our advantage. If the charge transfer reactions occurring at the surfaces could be tuned, it may be possible in the future to tailor these particles to various applications, including (but not limited to) nanodiamond-based drug nanomedicines to the individual needs of patients.
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In this study we have investigated the relationship between the localised surface charge transfer on a model diamond nanoparticle, as a function of the distribution and coverage of important intrinsic surface terminations: H, O and OH. We find that there are likely to be definite advantages to attempts to selectively passivating certain surface of diamond nanoparticles, depending on the type of reactions one wishes to encourage or suppress. 2. Computational methods Selective passivation of specific facets of nanoparticles is still challenging experimentally, so we have elected to use electronic structure simulations to model a range of different configurations, and compare the results. Specifically, we have used the self-consistent charge density functional tight-binding method (SCC-DFTB) [49] implemented in the DFTB + code [50], which is an approximate quantum chemical approach where the Kohn–Sham density functional is expanded to second order around a reference electron density obtained within the generalised gradient approximation (GGA). In this approach a confinement potential is optimised to anticipate the charge density and effective potential in both molecules and solids, and a minimal valence basis set is used to account explicitly for the two-centre tight-binding matrix elements within the DFT level. The double counting terms in the Coulomb and exchange–correlation potential, as well as the intra-nuclear repulsion are replaced by a universal short-range repulsive potential. In all the calculations, the “PBC” set of parameters is used to describe the contributions from diatomic interactions of carbon, hydrogen and oxygen [51]. This method has proven very successful in the past for modeling the absorption of oxygen containing groups on the surfaces of nanodiamond [52–54]. The diamond nanoparticle we have chosen in this case is a rhombitruncated-octahedron, as it is known to be a stable, low energy morphology, with a convenient combination of {100}, {111}, and {110} surfaces [55]. In particular, the (110) facets play an important role in the environmental stability and aggregation of diamond nanoparticles, and are resistant to the persistent graphitization that is characteristic of the bare (111) facets [56]. In this study, a 705 carbon atoms structure is used (denoted C705), which is 2.2 nm in average
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diameter, and to provide consistency with previous works [57], and a set of facet-dependent configurations have been considered. The configurations include structures where passivation is restricted to facets in a particular crystallographic orientation (denoted as {100}:X, {110}:X, and {111}:X, where X is the surface termination), and particles where passivation is suppressed on facets in a particular crystallographic orientation (denoted by the remaining facets that are passivated: {100}/ {110}:X, {111}/{110}:X, and {100}/{111}:X). The fully passivated (C705H228) particle has also been included for the purposes of comparison. All structures have been fully relaxed with a conjugate gradient methodology, without constraints, until forces on each atom were minimized to be less than 10−4 a.u. (i.e. ≈5 meV/Å). In the absence of surface terminations, this relaxation invokes surface reconstructions, to eliminate unpaired electrons where possible (see Fig. 1). At this point it is prudent to point out that the binding energy (and overall stability) of each type of surface termination depends on the surrounding thermochemical conditions [57], and the presence of excess charges [58]. This includes variations in temperature and the supersaturation of each species, and alternative species, in the bath. This issue has been treated in detail, using ab initio thermodynamics, and considering the impact of varying the partial pressure of water, in Ref. [59]. Following the relaxation of each structure the relative charge on the passivated carbon atoms and the attached atoms or molecules was extracted. There is no unique and definitive way of calculating charges, which are not strictly observable quantities, regardless of the method used. For this reason, it is preferable to compare the difference between charges on surfaces and molecules, again, regardless of the method used. In our particular case the Mulliken charges form an integral part of the energy functional in DFTB which expresses local density fluctuations around a given atom. The Mulliken charge fluctuations are calculated from the eigenvalue coefficients, and are algorithmically independent from bonding considerations and spatial partitioning schemes.[60] Therefore, although they generally have limited quantitative value, they are generated consistently and in a meaningful way (rather than using a separate method, a posteriori) and the relative differences are useful in illustrating trends, making them entirely suitable for comparing the impact of the different configurations included here.
Fig. 1. Schematic representation of the selectively passivated diamond nanoparticles used in this study. The orientation of the passivated facets is labelled, for hydrogen (top), oxygen (centre) and hydroxyl (bottom). The configurations are shown in increasing order of passivation (coverage) from left to right. Carbon atoms are shown in grey, oxygen in red, and hydrogen in pale green for clarity.
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3. Results and discussions During this analysis it was found that the surface carbon atoms can act as either donors or acceptors, depending upon the location within the particle (relative to the surface) and the type of surface termination. The magnitude of the charge transfer can vary enormously. The range of the net charges of H, O, and OH in each selective-passivation configuration is presented in Table 1, where positive value implies donating electrons (forming a positive residual charge on the host). Firstly, in the case of hydrogen (top panel of Table 1), we can see that although H always acts as an electron donor, the charge on H can vary between 0.038|e| and 0.078|e|. In general, the difference between the upper and lower values diminishes with increasing coverage, but difference of 0.012|e| is still present across the termination of a fully hydrogenated diamond nanoparticle. Similarly, although oxygen typically acts as electron acceptor (middle panel of Table 1), the net charge on O can vary between − 0.359 | e | and − 0.074 | e |. However in this case there is a relationship with the type of facets that are passivated or unpassivated, rather than simply the degree of coverage. Hydroxyl terminations always accept electrons from the nanoparticle, and the range of net charges lies between −0.190|e| and −0.072|e|. When we analyse the net charge distribution in greater detail we can see that these differences are functions of the positions of individual surface terminations with respect to the edges of the facets and the interactions (or lack thereof) between the neighbouring terminal groups. For example, if we consider the simple cases where the passivation is restricted to the (100), (111) or (110) facets, as shown in Fig. 2, some very simple trends emerge. In the absence of secondary inter-layer reconstructions, where C\\C bonds in the carbon surface are broken, as in the case of H (first column in Fig. 2) and the (100) facets, the magnitude of the net charge is greater near the periphery of the facet. At these positions the transfer of charge also involves C atoms at the edges, which also participate in neighbouring non-passivated facets. As we can see from the (100) facets (Fig. 2a) this holds regardless of whether the surface has a 2 × 1 or 1 × 1 reconstruction. On these facets the inter-surface termination interactions are also weak, as the binding sites are well separated. However, as the in-plane density of binding sites increases the intersurface termination interactions become greater, sub-surface reconstructions occur to compensate for the surface stress. This is most extreme in the case of the O-terminated (111) and (110) facets (centre Table 1 Range of net charges, Q, for the selectively passivated C705 diamond nanoparticle with X = H, O and OH. The configurations are shown in increasing order of coverage (top to bottom to each panel). Charges are in units of |e| (i.e. 1.602176565(35)×10−19 C), and the symbols max and min indicate the highest and lowest charge state, respectively. Configuration
Qmax X
Qmin X
Qmax C
Qmin C
{110}:H {100}:H {111}:H {100}/{110}:H {111}/{110}:H {100}/{111}:H Fully H-Passivated {110}:O {100}:O {111}:O {100}/{110}:O {111}/{110}:O {100}/{111}:O Fully O-passivated {110}:OH {100}:OH {111}:OH {100}/{110}:OH {111}/{110}:OH {100}/{111}:OH Fully OH-passivated
+0.078 +0.069 +0.063 +0.069 +0.058 +0.059 +0.050 −0.154 −0.263 −0.140 0.000 −0.137 −0.133 −0.074 −0.039 −0.117 −0.072 −0.049 −0.094 −0.089 −0.104
+0.056 +0.059 +0.046 +0.043 +0.041 +0.042 +0.038 −0.339 −0.295 −0.344 −0.359 −0.296 −0.330 −0.338 −0.174 −0.166 −0.187 −0.183 −0.175 −0.180 −0.190
+0.145 +0.163 +0.110 +0.150 +0.072 +0.063 +0.037 +0.452 +0.439 +0.448 +0.550 +0.403 +0.500 +0.402 +0.375 +0.260 +0.385 +0.376 +0.279 +0.389 +0.278
−0.250 −0.214 −0.184 −0.204 −0.092 −0.191 −0.071 −0.285 −0.279 −0.283 −0.433 −0.278 −0.366 −0.437 −0.345 −0.250 −0.367 −0.340 −0.153 −0.378 −0.131
of Fig. 2b and c, respectively) where oxygen dimers form, and the structure of the nanodiamond surface beneath is significantly distorted. In this case, the net charge on O atoms of the dimers is similar, and is approximately half the net change on a carbonyl oxygen; as would be expected. Similarly, on the OH-terminated (111) and (110) facets, the net charge on the OH groups are more consistent across the facet, except where sub-surface reconstructions have occurred, where the net charge is approximately halved. In both cases the sub-surface reconstructions occur near the periphery of the facet where the charge transfer involves C atoms at the edges with neighbouring non-passivated facets. While the charge on the surface termination is consistent with expectation, we can see that the underlying structure of the particle is very important. This leads to the issue of the net charge on the diamond nanoparticle, and here the situation becomes more complicated. The reconstructed surface on a small unpassivated (bare) diamond nanoparticle often exhibits a multi-polar surface electrostatic potential; particularly when the fraction of (111) surface area is high [61]. This phenomenon is not restricted to the surface, and subsurface atomic layers also accumulate a charge, but of alternating signs. This means that there is a net charge on the carbon atoms in both the outer atomic layer and the subsurface atomic layer, and in the absence of any passivation Qmax =0.152|e| and Qmin C C = − 0.214|e|. Returning our attention to Table 1 we can see that the values of Qmax C and Qmin change when parts (or all) of the surface are passivated. This is C due to secondary charge transfer which occurs between the two outermost layers to compensate for the donation or acceptance of electrons from the surface terminations. In the case of the H-passivation configurations, Qmin is characteristic of the outermost carbon layer, and Qmax is C C characteristic of the sub-surface layer. Here the range of variations in the net charges on the carbon atoms follows a similar trend to the terminal hydrogens; generally diminishing with increasing coverage; but the differences are more significant. At the lowest H-passivation configuration ({110}:H) the net charges on the C atoms are almost unperturbed, being almost the same as the unpassivated structure. Here we gain similar information by considering the net charges on the nanoparticle, or on the terminal hydrogen. In the case of the O- and OH-passivation configurations, Qmax is charC acteristic of the outermost carbon layer, and Qmin is characteristic of the C sub-surface layer. In these cases there is not the same relationship between the degree of coverage and the degree of charge transfer in these layers. Here we gain more information from the net charges on the terminal groups than on the nanoparticle. The relationship between configuration, coverage and net charges becomes apparent when we consider the average net charge of H, O, and OH in each configuration, as listed in Table 2, where positive value implies that the host nanoparticle donates electrons (retaining a residual positive charge). The ability of the diamond nanoparticle to accept charge from H on the surfaces is entirely coverage dependent, whereas the ability of the particle to donate charge to O or OH on the surface depends on both coverage, and the surfaces that are passivated. This suggests that in both cases an opportunity exists to tune the charge transfer to and from the particle, by controlling the species, coverage and distribution of surface groups. In order to see if these trends are consistently reflected in other indicative charge transfer properties we calculated the electron affinity (EA) and the ionization potential (IP) for the entire particle in each of these configurations. These properties are defined adiabatically with respect to the total energy of the neutral structure with N electrons E(N), and the corresponding anion E(N+ 1) and cation E(N −1), such that: EA ¼ EðNÞ−EðN þ 1Þ;
ð1Þ
and IP ¼ EðN−1Þ−EðNÞ:
ð2Þ
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Fig. 2. Examples of site-specific electron charge transfer to the terminal H (left), O (centre) and OH (right) surface terminations, on (a) the (100) surface, (b) the (111) surface, and (c) the (110) surfaces.
These energies are calculated directly using DFTB, since periodic boundary conditions are not applied and electrons can be added and subtracted without additional corrections. Once added (or subtracted) the charge distribution is optimised during the geometry relaxation, and the probability of the electron (or hole) residing at any given location is fractional. Table 2 Average net charge (unit: |e|) on H, O, and OH adsorbed selectively on the C705 diamond nanoparticle. Termination
Coverage (%)
〈QH〉
〈QO〉
〈QOH〉
{110}:X {100}:X {111}:X {100}/{110}:X {111}/{110}:X {100}/{111}:X Fully passivated
24.49 29.56 45.95 54.05 70.44 75.51 100.00
+0.069 +0.063 +0.055 +0.057 +0.049 +0.049 +0.043
−0.191 −0.275 −0.168 −0.216 −0.164 −0.194 −0.160
−0.103 −0.137 −0.187 −0.136 −0.142 −0.135 −0.142
As shown in Table 3, the IP of the nanoparticle is correlated with the degree of passivation, and all configurations exhibit a positive electron affinity, with the exception of the fully hydrogenated and fully hydroxylated particles. Negative electron affinities for these types of surfaces are well known for diamond; as mentioned above. However, the positive EA for diamond nanoparticles with incomplete H– or OH-passivation provides some new opportunities for driving different reactions under these conditions. In both cases simply detaching the H or OH from the minority {110} surfaces inverts the sign of the EA. While desorbing these passivants presents an entirely new experimental challenge (selectively, or not), these results suggest that incomplete passivation is not necessarily undesirable, and that driving charge transfer reactions in different directions (to, or from, the host nanoparticle) by changing the surface chemistry may be a real possibility. The advantage of this is, of course, that the surface chemistry can be engineered post synthesis, and does not necessitate modifications to the formation and purification processes already in place.
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Table 3 Electron affinity (EA) and ionization potential (IP) of the selectively H, O, and OH passivated diamond nanoparticle. Termination
{110}:X {100}:X {111}:X {100}/{110}:X {111}/{110}:X {100}/{111}:X Fully passivated
Coverage (%)
20.49 29.56 45.95 54.05 70.44 75.51 100.00
X=H
X=O
X = OH
EA
IP
EA
IP
EA
IP
3.510 3.365 3.972 2.898 2.463 2.982 −1.078
4.946 4.813 5.483 4.580 5.262 4.403 5.188
5.790 5.498 5.235 5.540 6.288 6.632 6.624
7.205 6.906 6.693 6.910 7.827 7.986 8.003
3.968 3.966 3.779 3.770 3.369 3.717 −0.249
5.358 5.391 5.144 5.202 6.154 5.111 5.800
4. Conclusion Based on the series of electronic structure simulations presented here we can extract a number of useful insights. Diamond nanoparticles can act as electron donors and/or electron acceptors, depending on the surface conditions. There is an intrinsic relationship between the type of surface terminations that are applied, the efficiency of the surface passivation (coverage) and the orientation of the facets where passivation is encouraged or suppressed. This clearly indicates the competition between saturated and unsaturated C sites on the surface, and this is also reflected in the global charge transfer properties of the entire particle. At this point it is worth pointing out that these systems share some similarities with large organic molecules, and an alternative analysis could be offered from this perspective. This would be an interesting topic for future work, as there may be new insights from the organic chemistry community that are opaque to a materials scientists and physicists in the field. We encourage researchers in this domain to take up the challenge, because if a means of controlling the location and concentration of surface terminations on diamond nanoparticles can be found, new methods for more tailored nanodiamond applications, such as drug delivery platforms, may follow. Acknowledgments Computational resources for this project have been supplied by the Australian National Computing Infrastructure national facility under Grant q27. References [1] B.Y.S. Kim, J.T. Rutka, W.C.W. Chan, Nanomedicine, N. Engl. J. Med. 63 (2010) 2434–2443. [2] G. Bao, S. Mitragotri, S. Tong, Multifunctional nanoparticles for drug delivery and molecular imaging, Annu. Rev. Biomed. Eng. 15 (2013) 253–282. [3] T.L. Doane, C. Burda, The unique role of nanoparticles in nanomedicine: imaging, drug delivery and therapy, Chem. Soc. Rev. 41 (2012) 2885–2911. [4] R. Bawa, Regulating nanomedicine - can the FDA handle it? Curr. Drug Deliv. 8 (2011) 227–234. [5] M.L. Etheridge, S.A. Campbell, A.G. Erdman, C.L. Haynes, S.M. Wolf, J. McCullough, The big picture on nanomedicine: the state of investigational and approved nanomedicine products, Nanomedicine 9 (2013) 1–14. [6] N. Kamaly, Z. Xiao, P.M. Valencia, A.F. Radovic-Moreno, O.C. Farokhzad, Targeted polymeric therapeutic nanoparticles: design, development and clinical translation, Chem. Soc. Rev. 41 (2012) 2971–3010. [7] E.C. Dreaden, A.M. Alkilany, X. Huang, C.J. Murphy, M.A. El-Sayed, The golden age: gold nanoparticles for biomedicine, Chem. Soc. Rev. 41 (2012) 2740–2779. [8] R.R. Arvizo, S. Bhattacharyya, R.A. Kudgus, K. Giri, R. Bhattacharya, P. Mukherjee, Intrinsic therapeutic applications of noble metal nanoparticles: past, present and future, Chem. Soc. Rev. 41 (2012) 2943–2970. [9] E.K. Chow, D. Ho, Cancer nanomedicine: from drug delivery to imaging, Sci. Transl. Med. 5 (2013), 216rv4. [10] C. Sun, J.S.H. Lee, M. Zhang, Magnetic nanoparticles in MR imaging and drug delivery, Adv. Drug Deliv. Rev. 60 (2008) 1252–1265. [11] M. Perfézou, A. Turner, A. Merkoçi, Cancer detection using nanoparticle-based sensors, Chem. Soc. Rev. 41 (2012) 2606–2622. [12] A.Z. Wilczewska, K. Niemirowicz, K.H. Markiewicz, H. Car, Nanoparticles as drug delivery systems, Pharmacol. Rep. 64 (2012) 1020–1037. [13] R. Mout, D.F. Moyano, S. Rana, V.M. Rotello, Surface functionalization of nanoparticles for nanomedicine, Chem. Soc. Rev. 41 (2012) 2539–2544. [14] D. Ho, Beyond the sparkle: the impact of nanodiamonds as biolabeling and therapeutic agents, ACS Nano 3 (2009) 3825–3829.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Tunable charge transfer on selectively functionalised diamond nanoparticles L. Lai a, A.S. Barnard b,⁎ a b
School of Physical Science and Technology, Southwest University, BeiBei District, Chongqing 400715, PR China CSIRO Virtual Nanoscience Laboratory, Parkville, VIC 3052, Australia
a r t i c l e
i n f o
Article history: Received 14 March 2016 Received in revised form 14 June 2016 Accepted 15 June 2016 Available online 17 June 2016 Keywords: Nanodiamond Bucky-diamond Ionization potential Electron affinity Charge transfer Simulation
a b s t r a c t For applications in nanomedicine, it has been shown that charge transfer properties are fundamental to the efficient loading and release of therapeutic agents, though it is not always clear whether intrinsic surface reconstructions and chemical conditions will encourage or suppress electron donation or acceptance. In this study we have investigated the relationship between the localised surface charge transfer, the ionization potential and the electron affinity of a model diamond nanoparticle, as a function of the distribution of important intrinsic surface terminations: H, O and OH. The detailed results reveal that diamond nanoparticles can act as electron donors or electron acceptors, depending on the type of surface terminations that are applied, the efficiency of the surface passivation (coverage) and the orientation of the facets where binding occurs. The results suggest that incomplete passivation is not necessarily undesirable, and that by driving charge transfer in different directions (to, or from, the host nanoparticle) by changing the surface chemistry new methods for more tailored applications such as drug delivery may be explored, based on post synthesis processes that do not necessitate modifications to the formation and purification processes already in place. Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.
1. Introduction Undoubtedly one of the most exciting applications for colloidal nanoparticles is in the field of nanomedicine [1–3]. Although there are still many challenges in realising this potential [4,5], there are some cases where research is progressing well toward the clinic [6,7], and others were even at the early stages we are beginning to see how the intrinsic properties of the nanoparticle are playing a crucial role [8,9]. Nanoparticles are useful for imaging [10], detection [11] and drug delivery [12]; provided their surfaces are dressed appropriately [13]. One example in this domain is diamond nanoparticles [14–17], which are already showing great promise for a range of therapies [19–21]. Benefits include increased selectivity [23,24] and sensitivity [22], the ability to more effectively moderate dosage [25] and provide more long term, sustained treatments without burst effusion [26]. It is now possible to produce stable suspensions of nanodiamonds with a hydrodynamic diameter of ~ 3.0 nm, with a narrow size distribution [27], which provides an even greater specific surface area for carrying therapeutic agents, biomarkers and ligands [28,29]. In each case, charge transfer properties are important to interactions of molecules with the surfaces of nanoparticles, including the loading and release of therapeutic agents, and will therefore impact the dose and dose rate during treatment. For example for the positively charged doxorubicin ions (NH3+) ⁎ Corresponding author. E-mail address: [email protected] (A.S. Barnard).
http://dx.doi.org/10.1016/j.diamond.2016.06.007 0925-9635/Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.
to be attracted to diamond nanoparticles the surfaces must be negatively charged, so that one needs to consider variations in the charge equilibrium on the surface sites and the amine group on doxorubicin to control the capture and release, as determined by solvent cations or anions [30]. In short, drugs may be bound to, and released by, the nanoparticle via charge transfer; which is well known in many other drug delivery platforms as well [31–34]. In general, the direction and efficiency of charge transfer depends on whether the underlying diamond nanoparticle acts as a donor or an acceptor, and can be characterized using the sign and value of the ionization potential (the donation of an electron) and the electron affinity (the accepting of an electron). It is possible to make direct measurements, and in the case of the H-terminated diamond surfaces it is well known that the electron affinity is negative [35,36]; though the exact value depends on a variety of factors [37–39]. This includes the type of interface [40,41], and species of surface terminations [43] or atmospheric conditions [42,44]. In addition to this, specific mixtures of surface groups (including H and OH) may result in NEA [45]. Similarly, recent studies have also shown that charge transfer at the nanodiamond surface depends sensitively on the types of particles, surfaces and surrounding medium [46–48]. Given these relationships, it is logical to assume it is possible to use them to our advantage. If the charge transfer reactions occurring at the surfaces could be tuned, it may be possible in the future to tailor these particles to various applications, including (but not limited to) nanodiamond-based drug nanomedicines to the individual needs of patients.
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In this study we have investigated the relationship between the localised surface charge transfer on a model diamond nanoparticle, as a function of the distribution and coverage of important intrinsic surface terminations: H, O and OH. We find that there are likely to be definite advantages to attempts to selectively passivating certain surface of diamond nanoparticles, depending on the type of reactions one wishes to encourage or suppress. 2. Computational methods Selective passivation of specific facets of nanoparticles is still challenging experimentally, so we have elected to use electronic structure simulations to model a range of different configurations, and compare the results. Specifically, we have used the self-consistent charge density functional tight-binding method (SCC-DFTB) [49] implemented in the DFTB + code [50], which is an approximate quantum chemical approach where the Kohn–Sham density functional is expanded to second order around a reference electron density obtained within the generalised gradient approximation (GGA). In this approach a confinement potential is optimised to anticipate the charge density and effective potential in both molecules and solids, and a minimal valence basis set is used to account explicitly for the two-centre tight-binding matrix elements within the DFT level. The double counting terms in the Coulomb and exchange–correlation potential, as well as the intra-nuclear repulsion are replaced by a universal short-range repulsive potential. In all the calculations, the “PBC” set of parameters is used to describe the contributions from diatomic interactions of carbon, hydrogen and oxygen [51]. This method has proven very successful in the past for modeling the absorption of oxygen containing groups on the surfaces of nanodiamond [52–54]. The diamond nanoparticle we have chosen in this case is a rhombitruncated-octahedron, as it is known to be a stable, low energy morphology, with a convenient combination of {100}, {111}, and {110} surfaces [55]. In particular, the (110) facets play an important role in the environmental stability and aggregation of diamond nanoparticles, and are resistant to the persistent graphitization that is characteristic of the bare (111) facets [56]. In this study, a 705 carbon atoms structure is used (denoted C705), which is 2.2 nm in average
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diameter, and to provide consistency with previous works [57], and a set of facet-dependent configurations have been considered. The configurations include structures where passivation is restricted to facets in a particular crystallographic orientation (denoted as {100}:X, {110}:X, and {111}:X, where X is the surface termination), and particles where passivation is suppressed on facets in a particular crystallographic orientation (denoted by the remaining facets that are passivated: {100}/ {110}:X, {111}/{110}:X, and {100}/{111}:X). The fully passivated (C705H228) particle has also been included for the purposes of comparison. All structures have been fully relaxed with a conjugate gradient methodology, without constraints, until forces on each atom were minimized to be less than 10−4 a.u. (i.e. ≈5 meV/Å). In the absence of surface terminations, this relaxation invokes surface reconstructions, to eliminate unpaired electrons where possible (see Fig. 1). At this point it is prudent to point out that the binding energy (and overall stability) of each type of surface termination depends on the surrounding thermochemical conditions [57], and the presence of excess charges [58]. This includes variations in temperature and the supersaturation of each species, and alternative species, in the bath. This issue has been treated in detail, using ab initio thermodynamics, and considering the impact of varying the partial pressure of water, in Ref. [59]. Following the relaxation of each structure the relative charge on the passivated carbon atoms and the attached atoms or molecules was extracted. There is no unique and definitive way of calculating charges, which are not strictly observable quantities, regardless of the method used. For this reason, it is preferable to compare the difference between charges on surfaces and molecules, again, regardless of the method used. In our particular case the Mulliken charges form an integral part of the energy functional in DFTB which expresses local density fluctuations around a given atom. The Mulliken charge fluctuations are calculated from the eigenvalue coefficients, and are algorithmically independent from bonding considerations and spatial partitioning schemes.[60] Therefore, although they generally have limited quantitative value, they are generated consistently and in a meaningful way (rather than using a separate method, a posteriori) and the relative differences are useful in illustrating trends, making them entirely suitable for comparing the impact of the different configurations included here.
Fig. 1. Schematic representation of the selectively passivated diamond nanoparticles used in this study. The orientation of the passivated facets is labelled, for hydrogen (top), oxygen (centre) and hydroxyl (bottom). The configurations are shown in increasing order of passivation (coverage) from left to right. Carbon atoms are shown in grey, oxygen in red, and hydrogen in pale green for clarity.
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3. Results and discussions During this analysis it was found that the surface carbon atoms can act as either donors or acceptors, depending upon the location within the particle (relative to the surface) and the type of surface termination. The magnitude of the charge transfer can vary enormously. The range of the net charges of H, O, and OH in each selective-passivation configuration is presented in Table 1, where positive value implies donating electrons (forming a positive residual charge on the host). Firstly, in the case of hydrogen (top panel of Table 1), we can see that although H always acts as an electron donor, the charge on H can vary between 0.038|e| and 0.078|e|. In general, the difference between the upper and lower values diminishes with increasing coverage, but difference of 0.012|e| is still present across the termination of a fully hydrogenated diamond nanoparticle. Similarly, although oxygen typically acts as electron acceptor (middle panel of Table 1), the net charge on O can vary between − 0.359 | e | and − 0.074 | e |. However in this case there is a relationship with the type of facets that are passivated or unpassivated, rather than simply the degree of coverage. Hydroxyl terminations always accept electrons from the nanoparticle, and the range of net charges lies between −0.190|e| and −0.072|e|. When we analyse the net charge distribution in greater detail we can see that these differences are functions of the positions of individual surface terminations with respect to the edges of the facets and the interactions (or lack thereof) between the neighbouring terminal groups. For example, if we consider the simple cases where the passivation is restricted to the (100), (111) or (110) facets, as shown in Fig. 2, some very simple trends emerge. In the absence of secondary inter-layer reconstructions, where C\\C bonds in the carbon surface are broken, as in the case of H (first column in Fig. 2) and the (100) facets, the magnitude of the net charge is greater near the periphery of the facet. At these positions the transfer of charge also involves C atoms at the edges, which also participate in neighbouring non-passivated facets. As we can see from the (100) facets (Fig. 2a) this holds regardless of whether the surface has a 2 × 1 or 1 × 1 reconstruction. On these facets the inter-surface termination interactions are also weak, as the binding sites are well separated. However, as the in-plane density of binding sites increases the intersurface termination interactions become greater, sub-surface reconstructions occur to compensate for the surface stress. This is most extreme in the case of the O-terminated (111) and (110) facets (centre Table 1 Range of net charges, Q, for the selectively passivated C705 diamond nanoparticle with X = H, O and OH. The configurations are shown in increasing order of coverage (top to bottom to each panel). Charges are in units of |e| (i.e. 1.602176565(35)×10−19 C), and the symbols max and min indicate the highest and lowest charge state, respectively. Configuration
Qmax X
Qmin X
Qmax C
Qmin C
{110}:H {100}:H {111}:H {100}/{110}:H {111}/{110}:H {100}/{111}:H Fully H-Passivated {110}:O {100}:O {111}:O {100}/{110}:O {111}/{110}:O {100}/{111}:O Fully O-passivated {110}:OH {100}:OH {111}:OH {100}/{110}:OH {111}/{110}:OH {100}/{111}:OH Fully OH-passivated
+0.078 +0.069 +0.063 +0.069 +0.058 +0.059 +0.050 −0.154 −0.263 −0.140 0.000 −0.137 −0.133 −0.074 −0.039 −0.117 −0.072 −0.049 −0.094 −0.089 −0.104
+0.056 +0.059 +0.046 +0.043 +0.041 +0.042 +0.038 −0.339 −0.295 −0.344 −0.359 −0.296 −0.330 −0.338 −0.174 −0.166 −0.187 −0.183 −0.175 −0.180 −0.190
+0.145 +0.163 +0.110 +0.150 +0.072 +0.063 +0.037 +0.452 +0.439 +0.448 +0.550 +0.403 +0.500 +0.402 +0.375 +0.260 +0.385 +0.376 +0.279 +0.389 +0.278
−0.250 −0.214 −0.184 −0.204 −0.092 −0.191 −0.071 −0.285 −0.279 −0.283 −0.433 −0.278 −0.366 −0.437 −0.345 −0.250 −0.367 −0.340 −0.153 −0.378 −0.131
of Fig. 2b and c, respectively) where oxygen dimers form, and the structure of the nanodiamond surface beneath is significantly distorted. In this case, the net charge on O atoms of the dimers is similar, and is approximately half the net change on a carbonyl oxygen; as would be expected. Similarly, on the OH-terminated (111) and (110) facets, the net charge on the OH groups are more consistent across the facet, except where sub-surface reconstructions have occurred, where the net charge is approximately halved. In both cases the sub-surface reconstructions occur near the periphery of the facet where the charge transfer involves C atoms at the edges with neighbouring non-passivated facets. While the charge on the surface termination is consistent with expectation, we can see that the underlying structure of the particle is very important. This leads to the issue of the net charge on the diamond nanoparticle, and here the situation becomes more complicated. The reconstructed surface on a small unpassivated (bare) diamond nanoparticle often exhibits a multi-polar surface electrostatic potential; particularly when the fraction of (111) surface area is high [61]. This phenomenon is not restricted to the surface, and subsurface atomic layers also accumulate a charge, but of alternating signs. This means that there is a net charge on the carbon atoms in both the outer atomic layer and the subsurface atomic layer, and in the absence of any passivation Qmax =0.152|e| and Qmin C C = − 0.214|e|. Returning our attention to Table 1 we can see that the values of Qmax C and Qmin change when parts (or all) of the surface are passivated. This is C due to secondary charge transfer which occurs between the two outermost layers to compensate for the donation or acceptance of electrons from the surface terminations. In the case of the H-passivation configurations, Qmin is characteristic of the outermost carbon layer, and Qmax is C C characteristic of the sub-surface layer. Here the range of variations in the net charges on the carbon atoms follows a similar trend to the terminal hydrogens; generally diminishing with increasing coverage; but the differences are more significant. At the lowest H-passivation configuration ({110}:H) the net charges on the C atoms are almost unperturbed, being almost the same as the unpassivated structure. Here we gain similar information by considering the net charges on the nanoparticle, or on the terminal hydrogen. In the case of the O- and OH-passivation configurations, Qmax is charC acteristic of the outermost carbon layer, and Qmin is characteristic of the C sub-surface layer. In these cases there is not the same relationship between the degree of coverage and the degree of charge transfer in these layers. Here we gain more information from the net charges on the terminal groups than on the nanoparticle. The relationship between configuration, coverage and net charges becomes apparent when we consider the average net charge of H, O, and OH in each configuration, as listed in Table 2, where positive value implies that the host nanoparticle donates electrons (retaining a residual positive charge). The ability of the diamond nanoparticle to accept charge from H on the surfaces is entirely coverage dependent, whereas the ability of the particle to donate charge to O or OH on the surface depends on both coverage, and the surfaces that are passivated. This suggests that in both cases an opportunity exists to tune the charge transfer to and from the particle, by controlling the species, coverage and distribution of surface groups. In order to see if these trends are consistently reflected in other indicative charge transfer properties we calculated the electron affinity (EA) and the ionization potential (IP) for the entire particle in each of these configurations. These properties are defined adiabatically with respect to the total energy of the neutral structure with N electrons E(N), and the corresponding anion E(N+ 1) and cation E(N −1), such that: EA ¼ EðNÞ−EðN þ 1Þ;
ð1Þ
and IP ¼ EðN−1Þ−EðNÞ:
ð2Þ
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Fig. 2. Examples of site-specific electron charge transfer to the terminal H (left), O (centre) and OH (right) surface terminations, on (a) the (100) surface, (b) the (111) surface, and (c) the (110) surfaces.
These energies are calculated directly using DFTB, since periodic boundary conditions are not applied and electrons can be added and subtracted without additional corrections. Once added (or subtracted) the charge distribution is optimised during the geometry relaxation, and the probability of the electron (or hole) residing at any given location is fractional. Table 2 Average net charge (unit: |e|) on H, O, and OH adsorbed selectively on the C705 diamond nanoparticle. Termination
Coverage (%)
〈QH〉
〈QO〉
〈QOH〉
{110}:X {100}:X {111}:X {100}/{110}:X {111}/{110}:X {100}/{111}:X Fully passivated
24.49 29.56 45.95 54.05 70.44 75.51 100.00
+0.069 +0.063 +0.055 +0.057 +0.049 +0.049 +0.043
−0.191 −0.275 −0.168 −0.216 −0.164 −0.194 −0.160
−0.103 −0.137 −0.187 −0.136 −0.142 −0.135 −0.142
As shown in Table 3, the IP of the nanoparticle is correlated with the degree of passivation, and all configurations exhibit a positive electron affinity, with the exception of the fully hydrogenated and fully hydroxylated particles. Negative electron affinities for these types of surfaces are well known for diamond; as mentioned above. However, the positive EA for diamond nanoparticles with incomplete H– or OH-passivation provides some new opportunities for driving different reactions under these conditions. In both cases simply detaching the H or OH from the minority {110} surfaces inverts the sign of the EA. While desorbing these passivants presents an entirely new experimental challenge (selectively, or not), these results suggest that incomplete passivation is not necessarily undesirable, and that driving charge transfer reactions in different directions (to, or from, the host nanoparticle) by changing the surface chemistry may be a real possibility. The advantage of this is, of course, that the surface chemistry can be engineered post synthesis, and does not necessitate modifications to the formation and purification processes already in place.
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Table 3 Electron affinity (EA) and ionization potential (IP) of the selectively H, O, and OH passivated diamond nanoparticle. Termination
{110}:X {100}:X {111}:X {100}/{110}:X {111}/{110}:X {100}/{111}:X Fully passivated
Coverage (%)
20.49 29.56 45.95 54.05 70.44 75.51 100.00
X=H
X=O
X = OH
EA
IP
EA
IP
EA
IP
3.510 3.365 3.972 2.898 2.463 2.982 −1.078
4.946 4.813 5.483 4.580 5.262 4.403 5.188
5.790 5.498 5.235 5.540 6.288 6.632 6.624
7.205 6.906 6.693 6.910 7.827 7.986 8.003
3.968 3.966 3.779 3.770 3.369 3.717 −0.249
5.358 5.391 5.144 5.202 6.154 5.111 5.800
4. Conclusion Based on the series of electronic structure simulations presented here we can extract a number of useful insights. Diamond nanoparticles can act as electron donors and/or electron acceptors, depending on the surface conditions. There is an intrinsic relationship between the type of surface terminations that are applied, the efficiency of the surface passivation (coverage) and the orientation of the facets where passivation is encouraged or suppressed. This clearly indicates the competition between saturated and unsaturated C sites on the surface, and this is also reflected in the global charge transfer properties of the entire particle. At this point it is worth pointing out that these systems share some similarities with large organic molecules, and an alternative analysis could be offered from this perspective. This would be an interesting topic for future work, as there may be new insights from the organic chemistry community that are opaque to a materials scientists and physicists in the field. We encourage researchers in this domain to take up the challenge, because if a means of controlling the location and concentration of surface terminations on diamond nanoparticles can be found, new methods for more tailored nanodiamond applications, such as drug delivery platforms, may follow. Acknowledgments Computational resources for this project have been supplied by the Australian National Computing Infrastructure national facility under Grant q27. References [1] B.Y.S. Kim, J.T. Rutka, W.C.W. Chan, Nanomedicine, N. Engl. J. Med. 63 (2010) 2434–2443. [2] G. Bao, S. Mitragotri, S. Tong, Multifunctional nanoparticles for drug delivery and molecular imaging, Annu. Rev. Biomed. Eng. 15 (2013) 253–282. [3] T.L. Doane, C. Burda, The unique role of nanoparticles in nanomedicine: imaging, drug delivery and therapy, Chem. Soc. Rev. 41 (2012) 2885–2911. [4] R. Bawa, Regulating nanomedicine - can the FDA handle it? Curr. Drug Deliv. 8 (2011) 227–234. [5] M.L. Etheridge, S.A. Campbell, A.G. Erdman, C.L. Haynes, S.M. Wolf, J. McCullough, The big picture on nanomedicine: the state of investigational and approved nanomedicine products, Nanomedicine 9 (2013) 1–14. [6] N. Kamaly, Z. Xiao, P.M. Valencia, A.F. Radovic-Moreno, O.C. Farokhzad, Targeted polymeric therapeutic nanoparticles: design, development and clinical translation, Chem. Soc. Rev. 41 (2012) 2971–3010. [7] E.C. Dreaden, A.M. Alkilany, X. Huang, C.J. Murphy, M.A. El-Sayed, The golden age: gold nanoparticles for biomedicine, Chem. Soc. Rev. 41 (2012) 2740–2779. [8] R.R. Arvizo, S. Bhattacharyya, R.A. Kudgus, K. Giri, R. Bhattacharya, P. Mukherjee, Intrinsic therapeutic applications of noble metal nanoparticles: past, present and future, Chem. Soc. Rev. 41 (2012) 2943–2970. [9] E.K. Chow, D. Ho, Cancer nanomedicine: from drug delivery to imaging, Sci. Transl. Med. 5 (2013), 216rv4. [10] C. Sun, J.S.H. Lee, M. Zhang, Magnetic nanoparticles in MR imaging and drug delivery, Adv. Drug Deliv. Rev. 60 (2008) 1252–1265. [11] M. Perfézou, A. Turner, A. Merkoçi, Cancer detection using nanoparticle-based sensors, Chem. Soc. Rev. 41 (2012) 2606–2622. [12] A.Z. Wilczewska, K. Niemirowicz, K.H. Markiewicz, H. Car, Nanoparticles as drug delivery systems, Pharmacol. Rep. 64 (2012) 1020–1037. [13] R. Mout, D.F. Moyano, S. Rana, V.M. Rotello, Surface functionalization of nanoparticles for nanomedicine, Chem. Soc. Rev. 41 (2012) 2539–2544. [14] D. Ho, Beyond the sparkle: the impact of nanodiamonds as biolabeling and therapeutic agents, ACS Nano 3 (2009) 3825–3829.
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