Organic functionalization of ultradispersed nanodiamond: Synthesis and applications

Article abstract of DOI:10.1039/b904302k

This work describes the chemical modification of ultradispersed nanodiamond results in the enrichment of the surface hydroxyl groups as by FT-IR, TGA, RGA-MS and XRD measurements. These hydroxyl groups can be conveniently functionalized with long chain al

Full text of DOI:10.1039/b904302k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Organic functionalization of ultradispersed nanodiamond: synthesis and
applications†
a
Wen-Wei Zheng, Yi-Han Hsieh, Yu-Chung Chiu, Sian-Jhu Cai, Chia-Liang Cheng and Chinpiao Chen
a
b
b
b
a
*
*
Received 6th March 2009, Accepted 9th September 2009
First published as an Advance Article on the web 2nd October 2009
DOI: 10.1039/b904302k
This work describes the chemical modification of ultradispersed nanodiamond results in the
enrichment of the surface hydroxyl groups as by FT-IR, TGA, RGA-MS and XRD measurements.
These hydroxyl groups can be conveniently functionalized with long chain alcohols (oxyhexanol) for
easy manipulation of different functional groups. The surface loading of the oxyhexanol groups were
found to be 0.13 mmol/g of nanodiamond. The functionalities on the surface of the modified
nanodiamond afford a new solid phase for the synthesis of peptides to facilitate the covalent
attachment of drug molecules. The conjugation of chiral ligands with a nanodiamond yields a new
enantioselective heterogeneous catalyst that exhibits moderate to good enantioselectivity in the
asymmetric aldol reaction.
with hydroxylated ND to introduce long alkyl groups in the
place of silane functional groups.20
Introduction
Nanoscale diamond particles are promising materials for study
owing to their relatively small and narrow size distribution,
tunable surface structures and chemical inertness, all of which
make them favorable new materials for electronic and biological
applications.1–4 Nanodiamond (ND) powder prepared by deto-
nation methods (ultradispersed diamond) exhibits unique and
interesting surface features. A uniform distribution of particle
sizes (2–10 nm), together with a variety of functional groups,
including carboxyl, lactone, ketone, hydroxyl and alkyl groups
are present on the surfaces of these ND particles and their
aggregates.5–12 Non-covalent or covalent modification of these
surface functional groups can change the properties of these
materials.13 Several chemical approaches were developed to
modify the surfaces of the nanodiamond powder by the covalent
attachment of various functional groups. For example, the
photolysis of perfluoroazooctane in the presence of nano-
diamond films on silicon substrate modified the surface by the
addition of perfluorooctyl functional groups.14–16 Similar modi-
fications were also made by radical and electrochemical reac-
tions. The reaction of nanodiamond powder with an elemental
fluorine/hydrogen mixture caused a high degree of fluorination
of the ND surface, yielding a fluoro-nanodiamond. Further
functionalization of these fluoro groups was performed with
alkyllithium reagents, diamines, and amino acids.17,18 Silane-
grafted nanodiamond particles were synthesized by the func-
tionalization of the hydroxylated diamond samples and
successfully applied as a novel solid phase in peptide synthesis.19
Alternatively, long chain carboxylic acid chlorides were reacted
This work describes the surface modification of nanodiamonds
by organic functionalization of the surface hydroxyl (primary
and secondary) groups. This method yields a ND surface that is
modified with several functional groups (halides, amines,
cyanide, azide and thiols), extending its potential range of
applications to new fields. The functionalized nanodiamond was
found to be a useful solid phase in peptide synthesis and allows
the covalent attachment of drug molecules and chiral ligands.
Results and discussion
Synthesis of alkylated nanodiamond
Commercially available synthetic detonation ND powder with
diameters 3–5 nm (Nanostructured and Amorphous Materials,
Inc. USA; >95% purity) was used in this study. Initial chemical
treatment of the ND powder by the typical oxidation method
was conducted to convert most of the surface functional groups
to carboxylic acids and hydroxyl groups (ND-1).21–23 The
homogeneity of the nanodiamond surface obtained by this
aDepartment of Chemistry, National Dong Hwa University, Shoufeng,
Hualien 974, Taiwan, Republic of China. E-mail: chinpiao@mail.ndhu.
edu.tw; Fax: +886-3-8630475
bDepartment of Physics, National Dong Hwa University, Hualien 974,
Taiwan, Republic of China
Scheme 1 Surface modification of detonation nanodiamonds. Reagents
and conditions: (a) (1) conc HCl/HNO3 (3:1); (2) 1 N NaOH (aq); (3) 1 N
HCl (aq); (b) (1) LiAlH4, THF; (2) 6 N NaOH (aq); (c) NaH, THF,
Cl(CH2)6OTHP; (d) p-TsOH, MeOH.
† Electronic supplementary information (ESI) available: Supplementary
spectra. See DOI: 10.1039/b904302k
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process was further enhanced by reducing the surface functional
groups with a suitable reducing agent.
Thermogravimetric (TGA) measurements of pristine and
ND-4 were carried out in order to support the covalent nature of
the grafting (Fig. 1). The residual gas analysis mass spectroscopy
(RGA/MS) observed that removal of the surface functional
groups only begins beyond 300 ꢀC, indicating a covalent bonding
of long tethered alcohols to the nanodiamond surface. The
weight loss corresponding to desorption of the adsorbed water
molecules and removal of organic groups amounts to 3.2%
and 4.1% respectively for ND-4. RGA/MS experiments on
ND-(O(CH2)6OH)x (ND-4) revealed fragments CH3OH (m/z 31)
and C4H8 (m/z 56) (Fig. 1b) at 340 ꢀC. Both fragments were
decomposed from the functionalized groups (–O(CH2)6OH) of
the nanodiamond surface. A similar observation favouring
covalent functionalization was reported earlier.4
Accordingly, air dried ND-1 was refluxed with LiAlH4 in THF
for a long time (Scheme 1), to convert the carboxylic acid groups
to easily functionalizable primary alcohols. The crude samples
obtained by reduction with LiAlH4 contained a large amount of
Al(OH)3 residues which were eliminated by stirring with 6 M
NaOH solution at 90 ꢀC overnight. The purity of ND-2 was
ascertained by energy dispersive X-ray spectroscopic (EDS or
EDX) analysis which indicated the absence of any aluminium
contamination in the sample validating the complete removal of
Al(OH)3 residues by alkali washings. The FT-IR spectrum of the
reduced ND powder (ND-2) confirmed the complete disappear-
ance of the carbonyl stretching frequency. However no hydroxyl
group could be clearly identified due to the interference of
surface-bound water molecules in the sample.
As further proof of the presence of free terminal hydroxyl
groups on the ND surface, ND-4 was deuterated with various
deuterated alkyl halides (Scheme 2). Treatment of ND-4 with
CD3I and C2D5I in the presence of sodium hydride yielded ND-5
and ND-6, which had intense C–D bands around 2067 cmꢁ1 and
2231 cmꢁ1, respectively, characteristic of the symmetric and
asymmetric C–D stretching frequency of CD3 groups (Fig. 2b
and 2c).24 The comparative experiments on nanodiamond
powder (ND-(OCD3)x) that was formed by the reaction of CD3I
with hydroxymethyl ND-2 (Fig. 2d) yielded similar observations.
The functionalization of nanodiamond studied in this work
presumably involves only the surface chemical bondings and
therefore does not influence the underlying crystalline structure.
For instance, the cubic XRD pattern of the nanodiamond
powder, presented in Fig. 3A, remains virtually unchanged by
the derivation reactions, as revealed by the XRD ‘‘fingerprints’’
of ND-(O(CH2)6OH)x (ND-4), which is taken as an example
(Fig. 3B). The surface derivatization is likely occurring from this
graphitic phase material.
The successful establishment of hydroxyl groups on the
nanodiamond surface was extended further by functionalization
with easily functionalizable groups having long tethers.
Williamson etherification of ND-2 with 6-(chloro-hexyloxy)-
tetrahydropyran afforded ND-3 possessing a six carbon tether
with a protected primary hydroxyl group. The formation of the
ether linkage (ND-3) was evident from their characteristic
stretching frequencies. The acid hydrolysis of the tetrahydropyr-
anyl ether of ND-3 yielded the key intermediate nanodiamond
powder (ND-4) with oxyhexanol groups on the ND surface.
The surface loading with oxyhexanol groups was estimated by
esterification with stearic acid followed by saponification; it was
found to be approximately 0.13 mmol/g of functionalized ND.
Fig. 1 (a) Thermogravimetric analysis (TGA) of: (i) nanodiamond ND,
(ii) functionalized nanodiamond ND-(O(CH2)6OH)x (ND-4); the weight
loss due to adsorbed water (first step) is (i) 1.5 wt%, (ii) 3.2 wt%; the
effective weight loss due to the removal of organic groups is (i) 3.2 wt%,
(ii) 4.1 wt%. (b) The residual gas analysis mass spectroscopy (RGA/MS)
of functionalized nanodiamond (ND-4). Real-time ion-monitoring plot
of pressure vs. time illustrating the conversion of ND-(O(CH2)6OH)x
(ND-4) into the ions CH3OH (m/z 31) and C4H8 (m/z 56).
Scheme 2 Functionalization of hydroxylated nanodiamond. Reagents
and conditions: (a) MsCl, Et3N, THF; (b) MX, DMF; (c) Na2S$9H2O,
DMF; (d) NH4OH, reflux; (e) NaH, THF, RX.
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Fig. 4 FT-IR spectra of functionalized ND measured in a vacuum: (a)
ND-(O(CH2)6F)x, (b) ND-(O(CH2)6CN)x, (c) ND-(O(CH2)6SH)x, (d)
ND-(O(CH2)6N3)x.
Fig. 2 Transmission FT-IR spectra of functionalized ND measured in
vacuum: (a) ND-(OH)m(CO2H)n (ND-1); (b) ND-(O(CH2)6OCD3)x
(ND-5); (c) ND–(O(CH2)6OCD2CD3)x (ND-6); (d) ND-(OCD3)x.
(ND-8) reveals a slight increase in the fluorine content, 1.9%,
indicating termination of the ND particle surface with fluorine.
Conjugation of functionalized nanodiamonds with drug molecules
Nanoparticles with covalently attached drug and bioactive
molecules as drug carriers have been extensively explored.25,26
Limited solubility, poor distribution, lack of selectivity, unfa-
vorable pharmacokinetics, and accidental damage to healthy
tissue associated with the conventional administering of drugs
are mitigated using nanoparticles that contain physically
entrapped or chemically bonded drug molecules.
Different nanoparticles have been successfully applied for this
purpose. Nanosized diamond–biomolecule complexes have been
recently explored as bioprobes and drug delivery agents.27 Such
usages are very promising because nanodiamond has little or no
adverse effect on living cells. The applications of nanodiamond
particles in these areas demand an effective protocol for conju-
gating drug molecules with the ND surface. Adsorption, covalent
and non-covalent immobilization have been widely employed to
modify ND surfaces with biologically active molecules.28–36 We
had already demonstrated that vitamin K3 and its analogues37
have favorable anticancer activity, and that, for example, the
2-hydroxyethylthio group on 2,3-bis(2-hydroxyethylthio)naph-
thalene-1,4-dione (1) can be functionalized with nanodiamond
powder (Scheme 3).
Fig. 3 XRD patterns of nanodiamond powder (A) and functionalized
nanodiamond ND-(O(CH2)6OH)x (ND-4) (B).
This low surface loading is comparatively lower than that
20
€
reported by Krueger.
Surface loading of nanodiamond with different functionalities
The successful grafting of the oxyhexanol groups to the ND
surface prompted the introduction of various functionalities on
the ND surface, which was achieved by converting the primary
hydroxyl of ND-4 to a good leaving group, followed by treatment
with different nucleophiles. Treatment of ND-4 with Et3N and
methanesulfonyl chloride yielded the mesylated nanodiamond
(ND-7) after the usual work-up procedure.
Two strategies were used to attach 1 to the nanodiamond. In
the first strategy, ND-13 with an ether link to 1 was synthesized
by the addition of ND-7 to the sodium salt of 1 formed with NaH
in anhydrous THF. In the other strategy, the drug molecule was
attached via an ester link (ND-15) to facilitate cleavage under pH
variations. The ester link was formed by the reaction of ND-7
with succinic anhydride, followed by the conversion of the
formed carboxylic acid groups to acid chloride and the reaction
with 1 in THF. The stretching of the carbonyl group in the FT-IR
Various metal salts were then heated together with ND-7 in
DMF, causing the displacement of the mesylate by the respective
nucleophiles (Fꢁ, CNꢁ, N3ꢁ, NH3, SHꢁ). Fig. 4 presents the
FT-IR spectra of all of the functionalized compounds. In all
cases, characteristic peaks of all functional groups were found: F
(1042 cmꢁ1), CN (2246 cmꢁ1), SH (616 cmꢁ1), and N3 (2101
cmꢁ1). The EDX analysis (not shown) of ND-(O(CH2)6F)x
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The cyclic voltammetric behaviors of ND, 1, and ND-13 were
determined using a glassy carbon electrode that was coated with
a film of ultrasonicated dispersed material (ND, 1, and ND-13) in
water. This film was slowly dried under vacuum and then covered
with 5% Nafion solution to fix the particles on the electrode.
Fig. 6 presents the obtained voltammograms. Two redox couples
whose reduction potentials are around ꢁ0.47 V and ꢁ0.33 V
versus SCE, corresponding to quinone to semiquinone and
semiquinone to hydroquinone reduction, were observed in the
voltammogram of 1. The data indicate that ND-13 displays
a similar voltammogram to 1 due to the naphthoquinone moiety,
and a slight positive shift in potential demonstrates that ND with
immobilized 1 is more susceptible to oxidation/reduction than 1,
itself, suggesting the effect of immobilization on the electro-
chemical behavior of 1.
Scheme 3 Derivatization of hydroxylated nanodiamond with vitamin
K3 analogue. Reagents and conditions: (a) NaH, 1, THF; (b) succinic
anhydride, Et3N, THF; (c) SOCl2, reflux; (d) 1, THF.
spectrum (Fig. 5e) clearly revealed the attachment of 1 to the
nanodiamond, such as ND-15.
Vitamin K3 and its derivatives with naphthoquinone moieties
are active electron acceptors in biological and chemical electron-
transfer reactions. A number of electrochemical studies of these
molecules have been reported.38–42 The covalent attachment of
1
to the nanodiamond surface was therefore identified
electrochemically.43 This finding is of interest because pristine
nanodiamond powder is reportedly electrochemically stable over
a wide range of potentials at very low background current.44,45
Fig. 5 Transmission FT-IR spectra of alkylated nanodiamond measured
inair:(a)ND-(O(CH2)6OMs)x (ND-7),(b)ND-(O(CH2)6OR)x (ND-13),(c)
ND-(O(CH2)6O2C(CH2)2CO2H)x (ND-14), (d) ND-(O(CH2)6O2C(CH2)2-
COCl)x, (e) ND-(O(CH2)6O2C(CH2)2COR)x (ND-15).
Fig. 6 Cyclic voltammograms of (a) ND; (b) 1, and (c) ND-13 on a glassy
carbon electrode in 0.05 M PBS (pH 7.0) at a scan rate of 0.2 mV Sꢁ1
.
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Fig. 7 Transmission FT-IR spectra of amino acid functionalised
nanodiamond measured in air: (a) ND-16, (b) ND-17, (c) ND-18, (d) ND-
19, (e) recovered ND-4.
advantages of such catalyst systems. A number of solid supports,
including organic polymers and inorganic solids that are conju-
gated with chiral ligands or metal complexes, yield a highly
enantioselective catalyst of many asymmetric reactions. The
efficient and easy functionalization of nanodiamonds motivated
the exploration of the activity of a chiral ligand-anchored
nanodiamond as a heterogenous catalyst. Since the pioneering
disclosure by List et al.50 that L-proline is an effective catalyst in
the intermolecular direct aldol reactions, the use of proline-based
chiral molecules as catalysts has received much attention.51
Consequently, L-proline-functionalized nanodiamonds were
synthesized and investigated as enantioselective catalysts.
Accordingly, L-4-hydroxyproline was covalently anchored onto
nanodiamond as presented in Scheme 5. The treatment of Boc-L-
hydroxyproline 3 with NaH followed by the addition of ND-7
afforded chiral ligand-tethered nanodiamond ND-20. Depro-
tection of the N-Boc under acidic conditions followed by
standard work up released the chiral heterogeneous ligand
ND-21. FT-IR spectra of ND-20 and ND-21 verified the presence
of the proline ligand on the nanodiamond.
Scheme 4 Direct synthesis of peptides on hydroxylated nanodiamond.
Reagents and conditions: (a) DCC, DMAP, N-(9-fluorenylmethoxy-
carbonyl)-L-phenylalanine, CH2Cl2; (b) piperidine (20%) in DMF; (c)
DCC, DMAP, N-(9-fluorenylmethoxycarbonyl)-L-valine, CH2Cl2; (d)
piperidine (20%) in DMF; (e) 1 M NaOH, rt; (f) Ac2O, 1 M NaOH;
benzyl alcohol, DCC, DMAP, CH2Cl2.
Functionalised ND as solid phase in peptide synthesis
The easy attachment and removal of molecules from the ND
surface yielded as a new solid phase from which is synthesized
amino acids and oligopeptides on the ND surface.
As a proof of this concept, the terminal hydroxyl group of
ND-4 was esterified with N-(9-fluorenylmethoxycarbonyl)-L-
phenylalanine using DCC as a peptide coupling agent (Scheme
4). Deprotection of the N-Fmoc using piperidine yielded free
valine bound to the alkylated nanodiamond. The above method
was used to attempt a second coupling with Fmoc-L-valine, to
yield a dipeptide. All the amino acid-bound ND surfaces were
identified by transmission FT-IR spectroscopy (Fig. 7), which
revealed a strong stretching frequency around 1700 cmꢁ1 asso-
ciated with an amide group, except in the case of the recovered
ND-4 (Fig. 7e). The dipeptide 2 was finally isolated by saponi-
fication of ND-19. Dipeptide 2 was derivatized to its benzyl ester-
acetamide and identified by NMR and IR.
The catalytic efficiency of ND-21 was tested in the model
reaction of p-nitrobenzaldehyde with cyclohexanone in the
presence of 40 wt% of catalyst under solvent-free conditions at
room temperature. Table 1 presents the results. This reaction
yielded the anti aldol product 4 in 5% isolated yield and in 73% ee
with the unreacted starting materials after 24 h. The reaction
Covalently anchored chiral proline on nanodiamond as new
enantioselective catalysts for asymmetric aldol reaction
The heterogenization of homogeneous catalysts has become an
important strategy for obtaining supported catalysts that retain
the active catalytic sites of a homogeneous analogue.46–49 The
ease of separation of the catalyst and reusability are the major
Scheme 5 Covalent attachment of L-hydroxyproline to nanodiamond.
Reagents and conditions: (a) NaH, ND-7, THF; (b) TFA–CH2Cl2 (1:4)
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Table 1 The direct aldol reaction of p-nitrobenzaldehyde and cyclo-
hexanone catalysed by ND-21 under different reaction conditions
Table 2 The aldol reaction of aromatic aldehydes with cyclohexanone
catalysed by ND-21 under solvent-free conditions
Entry Conditionsa
Yield (%)c ee (anti) (%)d
Ra
Yield (%)b
ee (anti) (%)c
25
[a]D (CHCl3)
Entry
1
2
3
4
5
6
7
8
Solvent-free; 24 h
5
6
8
8
9
11
11
8
6
7
73
77
61
67
61
41
47
55
29
61
65
63
71
82
79
79
79
1
2
3
4
5
6
o-NO2; 5
m-NO2; 6
m-CN; 7
p-CN; 8
p–Cl; 9
o-OMe; 10
9
8
5
4
4
2
+6.7 (c 0.20)
+11.5 (c 0.17)
+6.0 (c 0.08)
+8.4 (c 0.07)
+4.2 (c 0.09)
+7.5 (c 0.05)
87
85
71
73
66
69
Solvent-free; 48 h
Solvent-free; 72 h
Solvent-free; 96 h
Solvent-free; 120 h
Solvent-free; 144 h
Solvent-free; 168 h
DMSO; 48 h
CH2Cl2; 48 h
Toluene; 48 h
CH3CN; 48 h
DMF; 48 h
MgSO4; Solvent-free; 48 h
Stearic acid; Solvent-free; 48 h
Stearic acid/H2O; Solvent-free; 48 h 12
Benzoic acid; Solvent-free; 48 h 13
Benzoic acid/H2O;Solvent-free; 48h 11
a
b
2.0 equiv of cyclohexanone to aldehyde.
Enantiomeric excesses were determined by HPLC using chiracel OD–
Isolated yields.
9
c
10
11
12
13b
14 b
15 b
16 b
17 b
H column.
10
11
9
15
with >95% purity was purchased from Nanostructured and
Amorphous Materials, Inc., USA. Transmission FT-IR spectra
were obtained from KBr pellets using a Perkin Elmer Paragon
1000 FT-IR spectrometer, a JASCO FT/IR 410 spectrometer and
an ABB FTLA2000 FT-IR spectrometer with a MCT detector
(samples in vacuum, ꢂ10ꢁ6 torr). TGA measurements were made
in Al2O3 crucibles in an atmosphere of nitrogen at a heating rate
of 10 K minꢁ1 using a TGA/SDTA851e thermobalance. RGA
measurements were made using a DYCOR LC series Residual
Gas Analyzer (LC 200 series with UHV system, experiment
background pressure: 2 ꢃ 10ꢁ7 torr). Energy-dispersive X-ray
spectroscopy (EDS) was performed at SEM/EDS, JEOL JSM-
7000F. X-Ray diffraction analysis of the samples was carried out
using an X-ray diffractometer (XRD D8 Advanced, Bruker) with
Cu Ka radiation. X-Ray photoelectron spectroscopy (XPS,
Thermo K-Alpha) was carried out using an Al Ka radiation
source and electron energy analyzer (energy resolution, Ag 3d5/2
peak 0.5 eV, PET 0 ¼ C-0 peak 0.85 eV). The powder was pressed
a
b
2.0 equiv of cyclohexanone to aldehyde. 20 mol% additive; 0.5 mL
c
d
H2O. Isolated yields. Enantiomeric excesses were determined by
HPLC using chiracel OD–H column.
times were extended further to improve the yields of the product
(Table 1; entries 2–7). Longer reaction times, however, lowered
the enantioselectivity of the reaction with only very little
improvement in the isolated yields. In all cases, the diaster-
eoselectivity of the reaction could not be determined because of
the low yields of the products. The influence of the reaction
solvents on the conversion and enantioselectivity on the reaction
was explored by performing the reactions in various solvents. In
no case did the reaction proceed to completion, and the enan-
tioselectivities in all cases were much lower than that of the
solvent-free reaction (Table 1; entries 8–12). Additives such as
MgSO4, stearic acid52 and benzoic acid53 (with and without
water) increased enantioselectivity, although all reactions were
incomplete (Table 1; entries 13–17). The lower conversion
observed in all the cases could probably be due to a lower surface
loading of ND with the chiral ligand.
1
into an indium foil as a carrier for the measurements. H NMR
spectra were obtained on 400 MHz, and 13C NMR spectra were
obtained at 100.6 MHz using a Bruker NMR spectrometer.
Chemical shifts (d) are reported in ppm relative to CDCl3
(7.26 and 77.0 ppm respectively). Electrochemical measurements
were made using a model CHI420 electrochemical workstation
that was controlled by a personal computer. A three-electrode
system was used to make the measurements, with a bare glassy
carbon electrode as the working electrode, a saturated calomel
electrode (SCE) as the reference electrode and a platinum wire as
the auxiliary electrode. The measurements were made using
5.00 mL of buffer solutions. Enantiomeric excesses were deter-
mined using Lab Alliance Series III high performance liquid
chromatograph (HPLC) with a chiracel OD–H chiral column
(Daicel Chemical Industries, LTD). Optical rotations were
measured using a JASCO P-1010 polarimeter at the indicated
temperature using a sodium lamp (D line, 589 nm). Flash column
chromatography was performed using MN silica gel 60
(70–230 mesh) that was purchased from Macherey-Nagel.
Several aldehydes (5a–10a) were subjected to the aldol reaction
with cyclohexanone under solvent-free conditions in the presence
of the catalyst ND-21. Table 2 summarizes the results. All of the
substrates yielded the product with moderate to good enantio-
selectivity. As expected, electron-deficient aldehydes gave better
results than electron-rich aldehydes. Further exploration into the
use of catalyst ND-21 to improve the yields of aldol product and
the extended application to various substrates is under way.
Experimental
General
All commercial reagents were used without further purification.
Solvents were dried according to standard procedures. Raw ND
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Synthesis of ND-(OH)m(CO2H)n (ND-1). Synthetic ND
powders with particle diameters of 3–5 nm (Nanostructured and
Amorphous Materials, Inc. USA; >95% purity) were carboxyl-
ated/oxidized following the standard procedure. The ND sample
(1.0 g) was stirred in a 3:1 (v/v) mixture of concentrated HCl and
HNO3 (20 mL) at room temperature for three days, diluted with
deionized H2O, separated by centrifugation at 900 rpm, and
rinsed three times using deionized H2O. Following separation by
centrifugation at 900 rpm, the ND sample was heated in 0.1 M
NaOH solution at 90 ꢀC for 2 h. The reaction mixture was then
cooled to room temperature, diluted with deionized H2O, sepa-
rated by centrifugation, and rinsed three times using deionized
H2O. The ND sample was again heated in 0.1 M HCl at 90 ꢀC for
2 h, and washed as before until the washing solution became
weakly acidic. The resulting carboxylated/oxidized-ND (ND-1)
was separated and dried under vacuum.
Evaluation of the quantity of functional groups on the
nanodiamond
A solution of stearic acid (1.0 g) in thionyl chloride (3.0 mL) was
stirred under reflux for 6 h. The unreacted thionyl chloride was
removed by distillation, and the residue was dried under
a vacuum to yield stearoyl chloride. A suspended mixture of
ND-(O(CH2)6OH)x (ND-4) (1.0 g) and stearoyl chloride (1.0 g) in
THF (25 mL) was sonicated under argon for 5 min and stirred
overnight at room temperature. The reaction mixture was
quenched by adding deionized H2O, rinsing three times with
THF and twice with deionized H2O, and separation by centri-
fugation at 900 rpm. The resulting ND-stearoyl ester (IR, KBr
2924, 2853, 1717 cmꢁ1) was transferred to a round flask using
a small amount of deionized H2O and dried under a vacuum. A
suspended mixture of ND-stearoyl ester (1.0 g) in THF (20 mL)
was sonicated under argon for 5 min; 2 M NaOH (20 mL) was
added and the mixture was sonicated for 3 min before it was
refluxed for 24 h. The reaction mixture was cooled to room
temperature, rinsed three times with THF and twice with
deionized H2O; it was separated by centrifugation at 900 rpm. All
washings were collected, and the resulting ND-residue was
acidified by adding 1 M HCl, before rinsing three times with
THF and twice with deionized H2O; they were separated by
centrifugation at 900 rpm. All washings were combined and
acidified by adding 1 M HCl, before extraction with ethyl
acetate. The extracts were dried over anhydrous MgSO4, and
then filtered and concentrated to yield a solid. The resulting solid
was dissolved in a minimal amount of ethyl acetate, and then 1 M
NaOH was added until no more sodium salt was formed. The
sodium salt was collected by filtering; acidified by adding 1 M
HCl, and extracted using ethyl acetate. The extracts were dried
over anhydrous MgSO4, filtered and concentrated to form stearic
acid (35.7 mg, 39.6 mg, 37.9 mg).
Synthesis of ND-(OH)x (ND-2). A mixture of ND-(OH)m-
(CO2H)n (ND-1) (1.0 g) and THF (30 mL) was sonicated under
argon for 5 min, before LiAlH4 (1.0 g) was added and the system
was refluxed for 24 h. The reaction mixture was cooled to room
temperature and quenched with deionized H2O. The supernatant
liquid was removed by centrifugation at 900 rpm and the residue
was rinsed three times using deionized H2O. The residue was then
ꢀ
heated in 6M NaOH (60 mL) at 90 C overnight. The reaction
mixture was cooled to room temperature, washed and treated
with 0.1 M HCl, as described above. The repeatedly washed
sample was dried under vacuum at 50 ꢀC. IR (KBr): 3431, 2917,
1375 cmꢁ1. Elemental analysis (EDS): C: 94.72%, O: 4.02%.
(XPS): C: 93.99%, O: 2.97%, N: 1.81%, Cl: 0.89%, Na: 0.34%.
General procedure for the alkylation of ND-2
Therefore the amount of functional groups per gram of
functionalized nanodiamond was 0.13 mmol/g.
All of the reactions that involved alkylation using NaH as base
proceeded as follows. A suspended mixture of ND-(OH)x (ND-2)
(0.1 g) and THF (3 mL) was sonicated under argon for 5 min,
before NaH (0.1 g) was added, and the mixture was sonicated for
a further of 30 min. To this ND-(ONa)x mixture was added alkyl
halide (25 mL), and stirred at 45 ꢀC for 24 h. The reaction mixture
was cooled to room temperature, and quenched with deionized
water; the residue was separated and extensively rinsed with THF
and deionized H2O. The resulting alkylated nanodiamond was
transferred to a round flask using a small amount of deionized
H2O and dried under a vacuum.
Synthesis of ND-(O(CH2)6OMs)x (ND-7). Triethylamine
(1.0 mL) was added to a slurꢀry of ND-(O(CH2)6OH)x (ND-4)
(0.1 g) in THF (25 mL) at 0 C and stirred under argon for 30
min. Methanesulfonyl chloride (0.3 mL) was then added drop-
wise, and the resulting mixture was stirred at 0 ꢀC for 1 h and at
room temperature overnight. Deionized H2O was added, and the
centrifuged residue was washed repeatedly with THF and water.
The resulting ND-(O(CH2)6OMs)x (ND-7) was finally dried
under a vacuum. IR (KBr): 2918, 2850, 1336, 1217, 1169, 1048
cmꢁ1. Elemental analysis: C: 86.65%, H: 1.77%, N: 2.00%, O:
5.16%. XPS: C: 89.62%, O: 6.32%, N: 2.09%, S: 1.61%, Cl: 0.35%.
Synthesis of ND-(O(CH2)6OH)x (ND-4). A suspension of
ND-(O(CH2)6OTHP)x (ND-3) (0.5 g) in MeOH/H2O (3:1,
20 mL) was sonicated for 5 min; then p-TsOH was added until
the solution became acidic; it was then stirred at room temper-
ature overnight. The reaction mixture was cooled to room
temperature, rinsed three times with THF and twice with
deionized H2O, before being separated by centrifugation at
900 rpm. The resulting ND-(O(CH2)6OH)x (ND-4) was trans-
ferred to a round flask using a small amount of deionized H2O
Representative procedure for displacing meslyate (ND-7) with
various nucleophiles
Synthesis of ND-8.
A
suspended mixture of ND-
(O(CH2)6OMs)x (ND-7) (0.1 g) and DMF (5 mL) was sonicated
under argon for 5 min; NaF (0.02 g) was added and the mixture
was sonicated under argon for 3 min. It was then stirred at 70 ꢀC
for 12 h. The reaction mixture was cooled to room temperature
and rinsed three times with THF and twice with deionized H2O,
before separation by centrifugation at 900 rpm. The resulting
and dried under a vacuum. IR (KBr): 2923, 2859, 1036 cmꢁ1
.
Elemental analysis: C: 86.65%, H: 1.77%, N: 2.00%, O: 5.16%.
Elemental analysis (EDS): C: 96.10%, O: 3.64%. (XPS): C:
93.53%, O: 3.64%, N: 1.97%, Cl: 0.57%, S: 0.29%.
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ND-(O(CH2)6F)x (ND-8) was then dried under a vacuum. IR
(KBr): 2917, 2846, 1218, 1042 cmꢁ1. Elemental analysis (EDS):
C:93.48%, O: 4.17%, F: 1.90%. (XPS): C: 91.86%, O: 4.64%, N:
2.12%, S: 0.79%, F: 0.35%, Cl: 0.23%.
with foil. After stirring at room temperature for 24 h, deionized
H2O was added to quench the reaction, and rinsed three times
with THF and twice with deionized H2O; the system
was separated by centrifugation at 900 rpm. The resulting
ND-(O(CH2)6O2C(CH2)2COR)x (ND-15) was transferred to
a round flask using a small amount of deionized H2O before it
was dried under a vacuum. IR (KBr): 3422, 2918, 2856, 1719,
ND-9. IR (KBr): 2918, 2846, 2241 cmꢁ1. Elemental analysis
(XPS): C: 93.49%, O: 3.70%, N: 2.29%, S: 0.30%, Cl: 0.22%.
1225, 708 cmꢁ1
.
ND-10. IR (KBr): 2918, 2846, 2100 cmꢁ1. Elemental analysis
(XPS): C: 93.91%, O: 3.49%, N: 2.17%, S: 0.24%, Cl: 0.19%.
Synthesis of ND-16. To a solution of Fmoc-L-phenylalanine
(170 mg) in dichloromethane (40 mL) was added DCC (200 mg)
and DMAP (cat.). The solution was stirred at room temperature
for 5 min. Then, ND-(O(CH2)6OH)x (ND-4) (1 g) was added and
sonicated for 5 min. After it had been stirred at room tempera-
ture for 48 h, the reaction mixture was washed with dichloro-
methane (once), methanol (thrice), THF (twice) and water
(twice), with centrifugation at 900 rpm. The resulting ND-16 was
transferred to a round flask using a small amount of deionized
H2O and vacuum-dried. IR (KBr): 2917, 2851, 1711, 762,
ND-11. IR (KBr): 2917, 2846, 618 cmꢁ1. Elemental analysis
(XPS): C: 92.81%, O: 4.19%, N: 1.86%, S: 0.77%, Cl: 0.24%, Na:
0.14%.
Synthesis of ND-12.
A suspension mixture of ND-
(O(CH2)6OMs)x (ND-7) (0.3 g) and NH4OH (25 mL) was
sonicated under argon for 5 min and refluxed for 24 h. The
reaction mixture was cooled to room temperature, rinsed three
times with THF and twice with deionized H2O, before being
separated by centrifugation at 900 rpm. The resulting
ND-(O(CH2)6NH2)x (ND-12) was transferred to a round flask
using a small amount of deionized H2O, and dried under
a vacuum. IR (KBr): 2929, 1160 cm-1. Elemental analysis:
C: 87.45%, H: 1.78%, N: 2.18%, O: 5.66%. (XPS): C: 93.22%,
O: 3.93%, N: 1.86%, Cl: 0.49%, S: 0.51%.
743 cmꢁ1
.
Synthesis of ND-17. To a mixture of ND-16 (950 mg) in DMF
(20 mL) was added piperidine (4 mL); the mixture was then
sonicated for 5 min. After stirring at room temperature over-
night, the reaction mixture was washed with DMF (once),
methanol (twice), THF (twice), and water (twice) by centrifu-
gation at 900 rpm. The resulting ND-17 was finally vacuum-
Synthesis of vitamin K3 attached nanodiamond ND-13. A sus-
pended mixture of ND-(O(CH2)6OMs)x (ND-7) (0.1 g) and THF
(10 mL) was sonicated under argon for 5 min, before compound
1 (0.05 g) was added, and the mixture was stirred for 3 min. Then
sodium hydride (0.1 g) was added and protected from the light by
muffling up with foil. After stirring at room temperature for 24 h,
deionized H2O was added to quench the reaction; 1 M HCl was
added to acidify the solution weakly and the solid was rinsed
three times with THF and twice with deionized H2O; it
was separated by centrifugation at 900 rpm. The resulting
ND-(O(CH2)6OR)x (ND-13) was transferred to a round flask
using a small amount of deionized H2O before it was dried under
a vacuum. IR (KBr): 3420, 2918, 2850, 1521, 1456, 1215,
dried. IR (KBr): 2917, 2851, 1717, 702 cmꢁ1
.
Synthesis of ND-19. Coupling of Fmoc-L-valine to ND-17 and
the deprotection of the Fmoc group in the resulting dipeptide
ND-19 proceeded in the ways described for ND-16 and ND-17,
respectively. IR (KBr): 2917, 2851, 1725, 702 cmꢁ1
.
Synthesis of 2. A solution of ND-19 (750 mg) in 1 N NaOH
(20 mL) was sonicated for 5 min, and stirred at room
temperature overnight. The reaction mixture was washed with
methanol (twice) and water (twice), with centrifugation at 900
rpm. The resulting 2 was transferred to a round flask using
a small amount of deionized H2O, before being dried under
a vacuum. To a solution of 2 in 1 M NaOH (15 mL) was added
acetic anhydride (1 mL), and stirred at room temperature for 3
h. After the usual work up, the product (15 mg) was dissolved
in CH2Cl2 (5 mL), and sequentially added DCC (20 mg),
DMAP (2 mg) and benzyl alcohol (10 mg). The reaction
solution was stirred at room temperature for 24 h. After the
usual work up gave a benzyl ester-acetamide derivative of 2. 1H
NMR (400 MHz, CDCl3, d): 7.39–7.21 (m, 9H), 6.99–6.98 (m,
1H), 5.95–5.87 (m, 1H), 5.21–5.13 (m, 3H), 4.64–4.60 (m, 1H),
3.13–3.10 (m, 1H), 2.18–2.14 (m, 1H), 2.04–1.98 (m, 4H), 0.92–
0.90 (d, J ¼ 6.8 Hz, 3H), 0.87–0.85 (d, J ¼ 6.9 Hz, 3H). IR
(KBr): 3439, 3032, 2928, 2857, 1742, 1718, 1650, 1149, 1050,
718 cmꢁ1
.
Synthesis of ND-15.
A
suspended mixture of ND-
(O(CH2)6OH)x (ND-4) (0.1 g) and THF (10 mL) was sonicated
under argon for 5 min, before triethylamine (1 mL) and succinic
anhydride (0.1 g) were added, and the mixture was stirred at
room temperature overnight. 1 M HCl was added to acidify the
solution and rinsed three times with THF and twice with
deionized H2O; it was separated by centrifugation at 900 rpm.
The resulting ND-(O(CH2)6O2C(CH2)2CO2H)x (ND-14) was
transferred to a round flask using a small amount of deionized
H2O before it was dried under a vacuum. A suspended mixture of
ND-(O(CH2)6O2C(CH2)2CO2H)x (ND-14) (0.1 g) in SOCl2
(3 mL) was sonicated under argon for 3 min, and refluxed for 6 h.
The excess SOCl2 was removed by distillation and dried under
vacuum to give ND-(O(CH2)6O2C(CH2)2COCl)x. A suspended
mixture of ND-(O(CH2)6O2C(CH2)2COCl)x (0.1 g) and THF
(10 mL) was sonicated under argon for 5 min, before compound
1 (0.05 g) was added and protected from the light by muffling up
747, 700 cmꢁ1
.
Synthesis of ND-21. ND-20 was prepared from Boc-L-
hydroxyproline 3 and ND-7 following the general alkylation
procedure, which was described above. TFA–CH2Cl2 (10 mL,
1:4) was added to ND-20 (600 mg) and the mixture was sonicated
for 5 min. It was then stirred for 3 h at rt and then successively
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washed with CH2Cl2 (once), CH3OH–Et3N (49:1, thrice), THF
(twice) and finally deionized water (twice) with centrifugation.
The final product dried under vacuum to give ND-21 as a dry
powder. IR (KBr): 3400, 2917, 2849, 1703, 1341, 1216, 1156, 1107
cmꢁ1. Elemental analysis (XPS): C: 93.91%, O: 3.59%, N: 1.89%,
Cl: 0.60%.
3-[Hydroxy-(2-oxo-cyclohexyl)-methyl]-benzonitrile (7). Yield
5%; [a]25 +6.0ꢀ (c 0.08, CHCl3); dH(400 MHz; CDCl3; Me4Si)
D
7.63 (1H, s), 7.59–7.55 (2H, m), 7.47–7.43 (1H, t, JA,B 7.7), 4.82–
4.80 (1H, d, J1,2 8.5), 4.0 (1H, br s), 2.60–2.47 (2H, m), 2.40–2.33
(1H, m), 2.12–2.08 (1H, m), 1.84–1.81 (1H, m), 1.69–1.52 (3H,
m), 1.37–1.31 (1H, m); dC(100.6 MHz; CDCl3) 214.9, 142.6,
131.5, 130.7, 129.1, 118.7, 112.5, 74.0, 57.1, 42.6, 30.7, 27.6, 24.6;
IR (KBr) nmax/cmꢁ1 3508 (OH), 2946 (CH), 2861 (CH), 2228
(CN), 1695 (CO). HPLC analysis (Chiralcel OD–H column,
hexane:2-propanol ¼ 9:1, 0.5 ml/min, 254 nm UV detector)
71.1% ee, tR (major) ¼ 31.43 min, tR (minor) ¼ 40.38 min.
General procedure for asymmetric aldol reaction
Cyclohexanone (0.8 mmol) was added to ND-21 (25 mg, 40 wt%)
and sonicated for 30 s at rt. An aldehyde (0.4 mmol) was then
added to the reaction mixture, which was further sonicated for
30 min. The resulting mixture was stirred at rt for 48 h of
intermittent 30 min-long sonications every 2 h. After the indi-
cated reaction times (see Table 1), the reaction mixture was
diluted with ethyl acetate and centrifuged. The extraction was
performed twice. The combined organic phase was concentrated
under reduced pressure to yield a crude product which was
purified by column chromatography on silica-gel using ethyl
acetate–hexane as a mobile phase.
4-[Hydroxy-(2-oxo-cyclohexyl)-methyl]-benzonitrile (8). Yield
4%; [a]25 +8.4ꢀ (c 0.07, CHCl3); dH(400 MHz; CDCl3; Me4Si)
D
7.65–7.63 (2H, d, J1,2 8.2), 7.45–7.43 (2H, d, J1,2 8.1), 4.84–4.82
(1H, d, J1,2 8.4), 4.0 (1H, br s), 2.58–2.47 (2H, m), 2.39–2.33 (1H,
m), 2.13–2.09 (1H, m), 1.84–1.80 (1H, m), 1.68–1.53 (3H, m),
1.39–1.31 (1H, m); dC(100.6 MHz; CDCl3) 214.8, 146.3, 132.2,
127.7, 118.7, 111.7, 74.2, 57.1, 42.6, 30.7, 27.6, 24.7; IR (KBr)
n
max/cmꢁ1 3467 (OH), 2941 (CH), 2862 (CH), 2228 (CN), 1694
(CO). HPLC analysis (Chiralcel OD–H column, hexane:2-
propanol ¼ 9:1, 0.5 ml/min, 254 nm UV detector) 72.9% ee, tR
(major) ¼ 33.55 min, tR (minor) ¼ 50.19 min.
2-[Hydroxy-(4-nitro-phenyl)-methyl]-cyclohexanone (4). Yield
9%; [a]25 + 5.8ꢀ (c 0.14, CHCl3); dH(400 MHz; CDCl3; Me4Si)
D
8.21–8.19 (2H, d, J1,2 8.6), 7.51–7.49 (2H, d, J1,2 8.6), 4.90–4.88
(1H, d, J1,2 8.3), 4.07 (1H, br s), 2.62–2.56 (1H, m), 2.51–2.47
(1H, m), 2.38–2.31 (1H, m), 2.13–2.09 (1H, m), 1.84–1.81
(1H, m), 1.68–1.53 (3H, m), 1.39–1.35 (1H, m); dC(100.6 MHz;
CDCl3) 214.7, 148.3, 147.5, 127.8, 123.5, 74.0, 57.1, 42.6, 30.7,
27.6, 24.6; IR (KBr) nmax/cmꢁ1 3472 (OH), 2939 (CH), 2862
(CH), 1694 (CO). HPLC analysis (Chiralcel OD–H column,
hexane:2-propanol ¼ 9:1, 0.5 ml/min, 254 nm UV detector)
82.3% ee, tR (major) ¼ 34.79 min, tR (minor) ¼ 49.31 min.
2-[(4-Chloro-phenyl)-hydroxy-methyl]-cyclohexanone (9). Yield
4%; [a]25 + 4.2ꢀ (c 0.09, CHCl3); dH(400 MHz; CDCl3; Me4Si)
D
7.34–7.32 (2H, m), 7.27–7.25 (2H, d, J1,2 8.5), 4.78–4.76 (1H, d,
J1,2 8.6), 4.0 (1H, br s), 2.57–2.47 (2H, m), 2.39–2.35 (1H, m),
2.11–2.08 (1H, m), 1.82–1.79 (1H, m), 1.68–1.53 (3H, m), 1.34–
1.26 (1H, m); dC(100.6 MHz; CDCl3) 215.2, 139.5, 133.5, 128.5,
128.3, 74.1, 57.3, 42.6, 30.7, 27.7, 24.7; IR (KBr) nmax/cmꢁ1 3421
(OH), 2944 (CH), 2862 (CH), 1695 (CO). HPLC analysis
(Chiralcel OD–H column, hexane:2-propanol ¼ 9:1, 0.5 ml/min,
254 nm UV detector) 66.3% ee, tR (major) ¼ 18.49 min, tR
(minor) ¼ 26.89 min.
2-[Hydroxy-(2-nitro-phenyl)-methyl]-cyclohexanone (5). Yield
8%; [a]25 + 6.7ꢀ (c 0.20, CHCl3); dH(400 MHz; CDCl3; Me4Si)
D
7.85–7.83 (1H, d, J1,2 8.1), 7.77–7.75 (1H, d, J1,2 7.8), 7.64–7.61
(1H, t, JA,B 7.6), 7.44–7.40 (1H, t, JA,B 7.8), 5.44–5.43 (1H, d, J1,2
6.8), 4.18 (1H, br s), 2.77–2.72 (1H, m), 2.46–2.43 (1H, m), 2.38–
2.30 (1H, m), 2.10–2.07 (1H, m), 1.86–1.83 (1H, m), 1.75–1.56
(4H, m); dC(100.6 MHz; CDCl3) 214.9, 148.7, 136.6, 133.0, 129.0,
128.4, 124.1, 69.7, 57.3, 42.8, 31.1, 27.7, 24.9; IR (KBr) nmax/cmꢁ1
3431 (OH), 2949 (CH), 2862 (CH), 1702 (CO). HPLC analysis
(Chiralcel OD–H column, hexane:2-propanol ¼ 9:1, 0.5 ml/min,
254 nm UV detector) 87.3% ee, tR (major) ¼ 22.86 min, tR
(minor) ¼ 27.05 min.
2-[Hydroxy-(2-methoxy-phenyl)-methyl]-cyclohexanone (10).
Yield 2%; [a]25 + 7.5ꢀ (c 0.05, CHCl3); dH(400 MHz; CDCl3;
D
Me4Si) 7.40–7.38 (1H, m), 7.24–7.22 (1H, m), 6.99–6.95 (1H, t,
JA,B 7.4), 6.86–6.84 (1H, d, J1,2 8.2), 5.26–5.24 (1H, d, J1,2 8.3),
3.83 (1H, br s), 3.80 (3H, s), 2.74–2.69 (1H, m), 2.47–2.44 (1H,
m), 2.36–2.27 (1H, m), 2.07–1.95 (1H, m), 1.79–1.42 (5H, m);
dC(100.6 MHz; CDCl3) 215.5, 156.7, 129.6, 128.6, 127.7, 120.8,
110.4, 68.5, 57.3, 55.4, 42.5, 30.5, 27.9, 24.7; IR (KBr) nmax/cmꢁ1
3500 (OH), 2939 (CH), 2862 (CH), 1694 (CO). HPLC analysis
(Chiralcel OD–H column, hexane:2-propanol ¼ 9:1, 0.5 ml/min,
254 nm UV detector) 68.9% ee, tR (major) ¼ 19.75 min, tR
(minor) ¼ 25.77 min.
2-[Hydroxy-(3-nitro-phenyl)-methyl]-cyclohexanone (6). Yield
8%; [a]25D + 11.5ꢀ (c 0.17, CHCl3); dH(400 MHz; CDCl3; Me4Si)
8.20 (1H, s), 8.17–8.14 (1H, d, J1,2 8.1), 7.67–7.65 1H, (d, J1,2 7.6),
7.54–7.50 (1H, t, JA,B 7.8), 4.90–4.87 (1H, dd, JA,X 5.6, 2.7), 4.12–
4.11 (1H, d, J1,2 2.9), 2.64–2.59 (1H, m), 2.51–2.48 (1H, m), 2.42–
2.35 (1H, m), 2.14–2.09 (1H, m), 1.84–1.81 (1H, m), 1.69–1.55
(3H, m), 1.46–1.34 (1H, m); dC(100.6 MHz; CDCl3) 214.9, 148.2,
143.2, 133.2, 129.3, 122.9, 122.0, 74.0, 57.1, 42.6, 30.7, 27.6, 24.6;
IR (KBr) nmax/cmꢁ1 3478 (OH), 2941 (CH), 2865 (CH), 1698
(CO). HPLC analysis (Chiralcel OD–H column, hexane:2-
propanol ¼ 9:1, 0.5 ml/min, 254 nm UV detector) 85.7% ee, tR
(major) ¼ 28.62 min, tR (minor) ¼ 40.69 min.
Conclusions
A simple method for covalent functionalization and character-
ization of ultradispersed nanodiamonds is described. The process
involves the introduction of easily functionalizable primary
alcohol groups on the ND surface. Long tethers were attached to
facilitate the manipulation of functional groups on the nano-
diamond surface. Therefore, a variety of functional groups were
successfully formed on the nanodiamond surface, as evidenced
by spectral studies. The application of the functionalization
8440 | J. Mater. Chem., 2009, 19, 8432–8441
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Acknowledgements
We thank Dr Sankareswaran Srimurugan for his helpful
discussions (Postdoctoral fellowship awarded by National
Science Council of the Republic of China, under Contract NSC
96-2811-M-259-001). The authors thank the National Science
Council of the Republic of China, for financially supporting this
research under Contract NSC 96-2113-M-259-001 and NSC
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