Synthesis of water-soluble 2,2′-diphenyl-1-picrylhydrazyl nanoparticles: A new standard for electron paramagnetic resonance spectroscopy

Article abstract of DOI:10.1021/ja905395u

(Graph Presented) This communication reports a size-controlled synthesis of water-soluble 2,2′-diphenyl-1-picrylhydrazyl (DPPH) nanoparticles (NPs). These nanoparticles exhibit size-dependent absorption spectra and fast spin exchange-narrowed single-line

Full text of DOI:10.1021/ja905395u

Published on Web 08/12/2009
Synthesis of Water-Soluble 2,2′-Diphenyl-1-Picrylhydrazyl Nanoparticles: A
New Standard for Electron Paramagnetic Resonance Spectroscopy
Ou Chen, Jiaqi Zhuang, Fabrizio Guzzetta, Jared Lynch, Alexander Angerhofer,* and Y. Charles Cao*
Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611
Received June 30, 2009; E-mail: alex@chem.ufl.edu; cao@chem.ufl.edu
The discovery of size-dependent properties in inorganic colloidal
nanoparticles (NPs) has stimulated efforts to develop synthetic methods
for making NPs of small organics.1 Small-molecule organic NPs are
formed through noncovalent intermolecular interactions such as van der
Waals forces, H bonds, and π - π and solvophobic interactions.1 To date,
a variety of organic NPs have been synthesized using small molecules
possessing a rigid π system.1 These NPs exhibit interesting size-dependent
optical properties and are emerging as a new class of functional materials
with potential applications as key components in optoelectronics.1,2 Here
we report the synthesis and characterization of paramagnetic NPs made
of 2,2′-diphenyl-1-picrylhydrazyl (DPPH).
DPPH has historically played an important role in the development of
electron paramagnetic resonance (EPR) and has been just the second
organic free radical detected by EPR.3 Because of its single narrow
resonance line and its stability, DPPH is used as a standard field marker
for g-factor determination and magnetic scan calibration in both low- and
high-field EPR measurements4a and as a primary spin-concentration
standard in quantitative EPR spectrometry for the determination of free-
radical concentrations in various samples.4b However, the low solubility
of DPPH in water limits its application in aqueous solutions. To overcome
this limitation and allow the use of DPPH as a spin probe for aqueous
solutions, Tamano et al. recently developed an approach in which DPPH
is stabilized by encapsulation into aggregates of amphiphilic block
copolymers in water.5 The DPPH-containing polymer aggregates exhibit
single-line EPR spectra with linewidths of 5.0-15 G,5 which is much
broader than the typical line width of microcrystalline DPPH (∼1.5 G)
observed in the X-band.3,4 Here we report a colloidal synthesis of stable,
water-soluble DPPH NPs and show that the resulting particles exhibit
single-line EPR spectra with linewidths of ∼1.5-1.8 G, which are nearly
Figure 1. TEM images of DPPH NPs with diameters of (a) 90, (b) 200,
and (c) 310 nm. Scale bars are 500 nm. (d) Absorption spectrum of 250
nm DPPH NPs. Inset: peak position of the NP visible absorption band (II)
identical to those commonly observed for microcrystalline DPPH.
The synthesis of water-soluble DPPH NPs is based on a modified
reprecipitation method. In a reprecipitation-based synthesis, the
nucleation of organic NPs is initiated by a sudden introduction of
solvophobic interactions between the molecular building blocks (i.e.,
small-molecule precursors) and their surrounding solvent molecules;
this is achieved by the addition of a poor solvent (e.g., water) for
the molecular building blocks.1,6 The subsequent NP growth,
however, cannot simply be terminated by a temperature-quenching
process, as in advanced high-temperature syntheses of inorganic
nanocrystals, because this reprecipitation-based synthesis is nor-
mally carried out near room temperature. This difficulty has limited
the number of approaches available for size control of organic NPs.
In this work, we have found that gelatin, a common surfactant for
organic NPs, can rapidly terminate the growth of DPPH NPs in
water. Accordingly, we have demonstrated that size control of
DPPH NPs can be achieved simply by the injection of a gelatin
solution at a chosen particle-growth time. In contrast to previously
reported methods for making small-molecule NPs, in which control
of the final particle size is achieved by varying the concentrations
of precursors and surfactant molecules,1a,b the present method uses
only particle-growth time to control the final size of the NPs.
vs diameter (D). (e) EPR spectrum of 250 nm DPPH NPs (9.5 GHz, 298
K). Inset: NP EPR line width (LW) vs diameter.
In a typical experiment, DPPH (0.01 mmol) was dissolved in THF (1
mL) under Ar to form a deep-purple-colored stock solution, of which 100
µL was injected into a flask with 5 mL of water (Nanopure, 18.2 MΩ) at
room temperature with vigorous stirring. After a predetermined growth
time (0-2 h), aqueous gelatin solution (1.8 mL, 2 wt %) was injected
into the growth solution, which was kept under stirring for 5 more minutes;
the DPPH NPs were then isolated from the growth solution through
centrifugation. The resulting NPs were highly dispersible in water.
Transmission electron microscopy (TEM) showed that NPs with different
growth times (0-2 h) have diameters of 90-310 nm with a relative
standard deviation of ∼14% (Figure 1a-c). Also, these DPPH NPs exhibit
high stability in water, and no size-ripening was observed for over 6
months.
Electron diffraction showed that the as-prepared DPPH NPs possess
an amorphous structure (Figure S1 in the Supporting Information).7 The
DPPH NPs have absorption bands in the UV and visible regions [I and
II, respectively] (Figure 1d). These two bands originate from π-π*
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C O M M U N I C A T I O N S
transitions of DPPH radicals, and the delocalized radical electron makes
a major contribution to II.8 Both of the NP absorption bands exhibit a
size-dependent red shift relative to free DPPH molecules in THF: the
bigger the NPs, the larger the red shift, and vice versa (Figure 1d inset
and Figures S2 and S3). The red shift of these π-π* absorption bands is
due to J-type aggregation of DPPH molecules inside an NP.1,9 We attribute
the size-dependent red shift of these bands to stronger average intermo-
lecular interactions between DPPH molecules with increasing NP size,
as proposed by Yao and co-workers.1,9
A typical EPR spectrum of DPPH NPs consists of a characteristic
single narrow Lorentzian line (Figure 1e). The EPR line width is
weakly dependent on NP size. As the NP diameter decreases from
310 to 90 nm, the EPR line width increases from 1.5 to 1.8 G. The
single Lorentzian EPR line and narrow line width of these NPs are
due to the spin exchange-narrowing effect in the limit of fast
exchange.10 In this limit, the EPR lines of individual DPPH radicals,
which would otherwise be broadened by dipolar electron-electron and
electron-nuclear interactions, merge into a single Lorentzian line
whose width linearly decreases with increasing spin-exchange
interaction.4a,10 Thus, the slightly broader line width observed for 90
nm DPPH NPs is likely caused by a slower average spin-exchange
rate than in the bigger NPs. This slower exchange rate may be
associated with weaker average intermolecular interactions between
DPPH molecules in 90 nm NPs, as indicated by the peak position of
their absorption bands (Figure 1d inset and Figures S2 and S3).
their DPPH counterparts (Figure 2b), indicating that the presence of
DPPH-H does not substantially perturb the packing of DPPH molecules
inside the NPs.1,9 However, the location of DPPH-H significantly affects
the EPR line width of the NPs (Figure 2a). With DPPH-H in the shell,
the core/shell particles have an EPR line width of 1.7 G, identical to that
of pure DPPH particles of the same size. In contrast, the DPPH-H-doped
NPs show an EPR line width of 2.2 G, which corresponds to a reduction
of ∼30% in the spin-exchange interaction relative to that in the pure DPPH
NPs (Figure 2a).10 This result is likely due to the fact that the insertion of
DPPH-H molecules into the DPPH aggregates blocks the effective
exchange interaction between the DPPH radicals.10 In addition, the results
for the core/shell-type NPs further suggest that surface effects do not play
a major role in controlling the optical and paramagnetic properties of DPPH
NPs (Figure 2a,b).
To verify their suitability as EPR standards, we studied the stability
of DPPH NPs as a function of pH. The UV-vis absorption spectrum
(including the position and extinction of the two absorption bands), g
factor, EPR line width, and integrated EPR absorption intensity of the
DPPH NPs showed no measurable variation over a pH range of
3.0-10.0 (Figure 2c and Figure S5). These results show that these
NPs are stable under these conditions and therefore that these NPs are
practically useful as both a standard field marker and a primary spin-
concentration standard for aqueous samples over a wide pH range.4
In conclusion, we have reported a size-controlled synthesis of water-
soluble DPPH NPs, which exhibit size-dependent absorption spectra, fast
exchange-narrowed single-line EPR spectra with linewidths of ∼1.5-1.8
G, and stablility over a wide pH range. These properties make DPPH
NPs suitable for use as a new type of water-soluble EPR standard, which
is important for many applications in fields such as the food industry and
the life sciences.3,4 Furthermore, the DPPH NPs can potentially be used
as a spin probe in biomedical studies.3,5
Acknowledgment. Y.C.C. acknowledges the NSF (DMR-
0645520 Career Award) and ONR (N00014-06-1-0911). A.A.
acknowledges support from NHMFL UCGP and NSF (CHE-
0809725).
Supporting Information Available: Detailed synthetic procedures
and supporting figures. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
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Figure 2. (a) EPR spectra of DPPH NPs (blue), DPPH/DPPH-H core/
shell NPs (green), and DPPH-H-doped DPPH NPs (red). (b) Corresponding
absorption spectra of these NPs. (c) Integrated intensity of EPR absorption
(I) and g factor of DPPH NPs (310 nm diameter) as functions of pH. The
uncertainty in the g-factor determination was (0.0001, and the relative
uncertainty in the EPR intensity determination was ∼2.0%.
To further understand the spin-exchange interaction and J-type ag-
gregation of DPPH molecules inside the NPs, we designed and synthesized
three types of NPs: (1) DPPH, (2) core/shell particles with a DPPH core
coated with a shell of 2,2′-diphenyl-1-picrylhydrazine (DPPH-H), and (3)
DPPH particles doped with DPPH-H (Figure 2a).7 All of these NPs had
a nearly identical size of 180 nm (Figure S4); the core/shell and doped
NPs had a similar DPPH-H concentration of ∼20%. DPPH-H is a closed-
shell, reduced form of DPPH.11 Without the radical electron, DPPH-H
loses the visible band (II) of DPPH but maintains its UV band (I) at the
same wavelength and similar extinction.11 Indeed, because of the similar
concentration of DPPH-H components, the core/shell and doped NPs
exhibit nearly identical reduction in the intensity of their visible bands
(II) relative to that of their UV bands (I) (Figure 2b). The maxima of the
two bands in these two types of particles exhibit no shift from those in
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