RESEARCH PAPER

Stable gold nanoparticles obtained in pure acetone by laserablation with different wavelengths

Emilia Giorgetti • Maurizio Muniz-Miranda •

Paolo Marsili • David Scarpellini •

Francesco Giammanco

Received: 21 July 2011 / Accepted: 13 November 2011

� Springer Science+Business Media B.V. 2012

Abstract We prepared gold nanoparticles (NPs) by

ps laser ablation in pure acetone and water with 532

and 1,064 nm wavelengths. The NPs obtained in pure

acetone are stable for years and, depending on the

fabrication conditions, they can be very small, quasi

monodisperse and fluorescent. These properties are

not lost when they are transferred from acetone to

water. Post-irradiation tests of the colloids with

532 nm pulses, before and after phase transfer to

water, and surface enhanced Raman spectroscopy

(SERS), either on liquid and on dried samples, suggest

that the stabilization mechanism in acetone is related

to the light-induced formation on the gold surface of

enolate which, in some cases, can undergo degradation

with formation of amorphous carbon. Micro-SERS

tests were also used to demonstrate that functionali-

zation of the particles with 1,10-phenanthroline or

adenine is possible after transfer to the water phase,

which opens the way to the use of such structures for

biological and medical applications, such as biocom-

patible fluorescent or Raman markers.

Keywords Gold nanoparticles � Laser ablation �SERS � Fluorescence

Introduction

The ultra fast development of the possibilities of

matter manipulation down to the nanoscale, which

occurred in the past 20 years, makes now available

several types of metal/dielectric nanostructured mate-

rials. Among them, nanoparticles (NPs) of coinage

metals and, above all, of gold, represent an important

class, largely investigated up to date, due to their

perspective applications to several fields, including

medical diagnostic, sensing, drug delivery, and devel-

opment of smart materials (Jain et al. 2006, 2009;

Kumar et al. 2008 and references therein). In spite of

Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-011-0648-9) containssupplementary material, which is available to authorized users.

E. Giorgetti (&)

INSTM and Istituto dei Sistemi Complessi, Consiglio

Nazionale delle Ricerche, Via Madonna del Piano 10,

Sesto Fiorentino, 50019 Florence, Italy

e-mail: emilia.giorgetti@fi.isc.cnr.it

M. Muniz-Miranda

Dipartimento di Chimica ‘‘Ugo Schiff’’, Universita di

Firenze, Via della Lastruccia 3, 50019 Florence, Italy

M. Muniz-Miranda

European Laboratory for Non-linear Spectroscopy

(LENS), Via N. Carrara 1, 50019 Florence, Italy

P. Marsili � D. Scarpellini

Istituto dei Sistemi Complessi, Consiglio Nazionale delle

Ricerche, Via Madonna del Piano 10, Sesto Fiorentino,

50019 Florence, Italy

F. Giammanco

Department of Physics ‘‘E. Fermi’’, University of Pisa,

Largo Bruno Pontecorvo 3, 56127 Pisa, Italy

123

J Nanopart Res (2012) 14:648

DOI 10.1007/s11051-011-0648-9

the high degree of versatility, the currently used

preparation methods based on chemical reduction are

seldom compatible with applications to biology or

nanomedicine, due to the presence of contaminants

coming from reaction by-products (Chen et al. 2003;

Alloisio et al. 2009; Seo et al. 2006; Loo et al. 2005;

Chen et al. 2005; Trigari et al. 2011). In this sense, the

method of pulsed laser ablation of a metallic target in

liquid environment, which has been widely studied for

at least 10 years (see for example Amendola and

Meneghetti 2009 and references therein), although

more expensive, slow and less versatile in terms of

particle shape, is extremely simple, fully compatible

with different solvents and, above all, it exhibits the

enormous advantage of the purity of the final products.

Beyond purity, important issues concerning the

preparation of metallic NPs and their subsequent

application are the long-term stability and the possi-

bility of surface functionalization. Although high

long-term stability can be obtained by using proper

capping agents, as sodium-dodecyl-phosphate (SDS)

or ethylendiamine-core poly(amidoamine) (PAMAM-

G5) (Mafune et al. 2001; Giusti et al. 2007), their

presence on the particle surface could impair adsorp-

tion of functionalization compounds. Therefore, it is

important to investigate the possibility to obtain stable

metal NPs in pure solvents.

In the case of gold, by properly choosing the

working conditions, it is possible to obtain AuNPs

which are stable for months in pure water (Silvestre

et al. 2004). However, it is still to be clarified how

preparation conditions (laser wavelength, energy of

the single pulse, fluence, focusing conditions, atmo-

sphere, temperature, and pH), characteristics of the

suspension (concentration, particle dimensions) and

post-irradiation for reshaping purposes can influence

the long-term stability of these products (Giammanco

et al. 2010; Werner et al. 2008). In contrast, acetone

seems to be a more promising solvent for the

preparation of stable uncapped AuNPs. There are

several reports in the literature describing the use of

acetone to obtain colloidal suspensions of uncapped

Au or Ag NPs with high long-term stability (Kawasaki

and Nishimura 2006; Kazakevich et al. 2006; Tilaki

et al. 2006; Tarasenko et al. 2005; Bozon-Verduraz

et al. 2003, Burakov et al. 2005). In the paper by

Kawasaki and Nishimura (2006), where laser ablation

of Au flakes in pure acetone is described, the authors

speculate that the stabilization would be due to a

charge transfer between the C=O group of the solvent

and the metal. Although some hypotheses have been

made, an investigation on the mechanism at the basis

of the stabilization process is still lacking. Further-

more, direct application of such products to drug

delivery or in vivo diagnostic is impaired by the

presence of acetone. Therefore, in order to increase the

biocompatibility of the samples, it is important to

assess the possibility of their phase transfer to water

for subsequent functionalization (Boisselier and

Astruc 2009).

In this article, we describe the preparation of

AuNPs by laser ablation in pure acetone. This article

provides a spectroscopic and morphologic character-

ization and a comparison with analogous systems

obtained in the same experimental conditions, but in

different pure solvents, namely deionized water and

ethanol. A Raman study on the samples of AuNPs in

acetone is carried out both to assess the stabilization

mechanism and, after phase transfer to water, to check

the possibility of functionalization of the metallic

surface. Lastly, post-irradiation experiments with

532-nm pulses are reported to elucidate the solvent

role on the stability of the colloids.

Experimental methods

We prepared AuNPs by laser ablation of a gold target

in liquid environment with the fundamental

(1,064 nm) or the second harmonic (532 nm) of a

mode-locked Nd-YAG laser (EKSPLA PL2143A: rep.

rate 10 Hz, pulse width 25 ps at 1064 nm and 20 ps at

532 nm). The laser beam was focused with a lens

having 20 cm focal length. The target was placed in a

1 cm 9 1 cm-quartz cuvette and was kept 2 cm in

front of the focal plane. The diameter of the laser spot

on the target was fixed at 1.4 mm. We also performed

post-irradiation tests by illuminating some of the

obtained suspensions in absence of the target with the

second harmonic from the same laser. The pulse

energy (15 mJ) and the focusing conditions of the

laser beam were maintained constant through all

ablation and post-irradiation experiments. The vol-

umes of the liquid used for the ablation and post-

irradiation tests were 2 and 1 ml, respectively.

The ablation process or the post irradiation of the

NPs were monitored by measuring in situ the visible

spectra with an Ocean Optics fibre spectrophotometer

Page 2 of 13 J Nanopart Res (2012) 14:648

123

and a deuterium–tungsten lamp. The sampling beam

was perpendicular to the ps laser beam and crossed the

quartz cuvette 0.5 cm above the bottom of the cell. We

also recorded UV–Vis spectra some days after the

preparation of the suspensions with a double beam

spectrophotometer (Perkin Elmer mod. Lambda19)

and fluorescence spectra with a Jasco FP-750

spectrofluorimeter.

AuNPs suspensions were prepared in deionized water

(18.2 MX cm at 25�C), ethanol (Carlo Erba [ 99.8%),

or acetone (Carlo Erba [ 99.8%). The gold target was

purchased from Goodfellow (high purity: 99.95%).

We transferred AuNPs obtained in pure acetone

into deionized water, by adding water to the colloids

(*2:1 or 1:1 acetone/water volume ratio) and letting

acetone to evaporate. We performed phase transfer at

room temperature under fume-hood. The process

typically takes several hours. We functionalized

aqueous colloids with 1,10-phenanthroline (phen) or

adenine by diluting a water solution of phen or adenine

with the Au hydrosol, in order to achieve around

10-4 M concentration of ligands.

TEM samples were obtained by dipping carbon-

coated copper grids in the suspensions and the images

were recorded with a Philips CM12, 120 kV. Particle

mean diameter and dispersivity were determined by

fitting the measured statistical distributions by an

asymmetric Gaussian, i.e., a Gaussian with different

standard deviations below (r-) and above (r?) the

average particle diameter (Giorgetti et al. 2011).

The f-potential was measured with a Zetasizer

Nano ZS90, Malvern Instruments. In this apparatus,

the f-potential is automatically calculated from the

electrophoretic mobility on the basis of the Helm-

holtz–Smoluchowski relation.

The surface enhanced Raman spectra (SERS) of Au

colloids were recorded using the 514.5 nm line of a

Coherent Argon ion laser (power: 50 mW), a Jobin-

Yvon HG2S monochromator equipped with a cooled

RCA-C31034A photomultiplier and a data acquisition

facility. Power density measurements were performed

with a power meter instrument (model 362; Scientech,

Boulder, CO, USA), giving *5% accuracy in the

300–1000 nm spectral range.

The SERS spectra of dried Au NPs were measured

using a Renishaw RM2000 micro-Raman apparatus,

coupled with a diode laser source emitting at 785 nm

or a laser source emitting at 514.5 nm. Sample

irradiation was accomplished by using the 950

microscope objective of a Leica Microscope DMLM.

The beam power was *3 mW, the laser spot diameter

was adjusted between 1 and 3 lm. Raman scattering

was filtered by a double holographic Notch filters

system and collected by an air cooled CCD detector.

The acquisition time for each measurement was 10 s.

All spectra were calibrated with respect to a silicon

wafer at 520 cm-1.

Results and discussion

Preparation of AuNPs in pure solvents

Figure 1 shows typical UV–Vis spectra obtained with

infrared (1,064 nm) and green (532 nm)-light ablation

in three different pure solvents. In all cases, we kept

the energy of the single pulse constant at 15 mJ and

stopped the ablation when the same level of absor-

bance in the plasmon peak was reached, namely, 1.1

and 0.9 in the infrared (IR) and green cases, respec-

tively. The spectra were taken in situ during the

ablation and those reported in the figure represent the

last recording after laser switch-off.

While all spectra of Fig. 1a are identical, big

differences are observed among those of Fig. 1b. The

spectra pertaining to pure water exhibit a well resolved

plasmon band and no major changes with ablating

wavelength (gray lines in Fig. 1a, b). In contrast,

acetone and ethanol undergo big changes. In both

cases, the poor resolution of the plasmon band in

Fig. 1b suggests that green-light ablation in acetone

and ethanol produces much smaller particles with

respect to IR-light ablation. Furthermore, in the case of

ethanol (dashed black line in Fig. 1b), the appearance

of an absorption tail in the red, during the ablation

process itself, is evidence of particle aggregation and

instability. As a matter of facts, uncapped AuNPs in

ethanol rapidly precipitate after fabrication. This is the

reason why, in the following, we will limit ourselves to

description of results obtained with water and acetone.

The previous considerations were confirmed by

TEM analysis. In the case of IR-light ablation, TEM

images obtained with the three solvents are quite

similar. The particles are spherical and polydispersed,

with a non negligible amount of particles with

diameter of the order of 10 nm or more. In the case

of water, reported in Fig. 2a, the particle mean

diameter obtained with IR light is 4.5 nm, and the

J Nanopart Res (2012) 14:648 Page 3 of 13

123

statistical distribution is fitted by a strongly asymmet-

ric Gaussian with r- = 1.4 nm and r? = 12 nm. The

polydispersivity can be considerably reduced by

fabrication with 532 nm light. In this case, the results

for water are reported in Fig. 2b. The main difference

with respect to Fig. 2a is in the absence of ‘‘big’’

particles, which results in a narrowing of the statistical

distribution. Indeed, the mean diameter keeps 4.5 nm,

but the distribution is fitted by a symmetric Gaussian

with r = 1.5 nm. Another difference is represented

by the formation of particle aggregates, which the

circles in Fig. 2b put in evidence. In the light of

several studies on the interaction between AuNPs and

short pulses at 532 nm (Giammanco et al. 2010 and

references therein), such features are not surprising.

Indeed, high intensity green-light irradiation of AuN-

Ps, either during the production process itself or,

subsequently, due to accidental post-irradiation of

particles which happen to be in the path of the ablating

beam, can lead to electron ejection and, in general, to a

strong perturbation of the surface electrostatic equi-

librium, which affects particle growth and stability

(Giammanco et al. 2010; Werner et al. 2008).

In the case of acetone, the particles obtained with IR

ablation (Fig. 2c) exhibit 4 nm average diameter and

the statistical distribution is well fitted by an asym-

metric Gaussian with r- = 2 nm and r? = 5.2 nm.

As expected on the basis of the spectrum of Fig. 1b,

the NPs obtained by green-light ablation are much

smaller. Although the resolution of our TEM equip-

ment impaired an accurate evaluation of average

diameter and statistical distribution, however, the

particles dimensions appear quite uniform and around

2.5–3 nm (Fig. 2d). Such samples are extremely

stable, with no evidence of spectral changes, aggre-

gation or formation of precipitate over periods longer

than 2 years.

Figure 3 shows the effect of green-light post-

irradiation on AuNPs in pure acetone, namely on the

samples of Fig. 2c, d. In general, it is well known that

when stabilizer-capped Ag or AuNPs are exposed to

532 nm laser pulses ‘‘permanent’’ photofragmentation

occurs, namely laser pulses fragment the particles and,

due to the presence of excess stabilizer in suspension,

the fragments cannot reaggregate. In this way, it is

possible to reach shot-by-shot thorough bleaching of a

suspension, down to dissolution of gold and disap-

pearance of the interband absorption (Giorgetti et al.

2011). Such processes are well known and are due to

the coexistence of several entangled effects related to

the resonance of the impinging wavelength with the

absorption spectra of the metal NPs, i.e., heating with

subsequent melting or evaporation, and multiphoton

extraction of electrons with subsequent Coulomb

explosion (Kamat et al. 1998; Mafune et al. 2002;

Giammanco et al. 2010). The NPs of Fig. 3 are

uncapped. This means that, in principle, fragments can

reaggregate (Boyer et al. 2010) and form new NPs,

whose dimensions can be different from the initial

ones, or collapse due to perturbation of the electro-

static equilibrium among particles (Werner et al.

2008).

According to Fig. 3, the spectrum of the sample

obtained by 1,064 nm ablation (thick black line)

underwent dramatic changes after 2,400 laser shots

(thin black line), with almost thorough suppression of

the plasmon band. Such spectrum is typical of very

small particles, whose diameter does not reach the

0.0

0.2

0.4

0.6

0.8

1.0(a)

(b)

ethanol

water

acetone

1064nm

abso

rban

ce

wavelength (nm)400 500 600 700 800

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 ethanol

water

acetone

532nm

abso

rban

ce

wavelength (nm)

Fig. 1 UV–Vis spectra of suspensions of AuNPs obtained in

pure water (solid gray lines), ethanol (dashed black lines), or

acetone (solid black lines). a Ablation with 1064 nm and 15 mJ

per pulse; b ablation with 532 nm and 15 mJ per pulse. Optical

path length (OPL) = 1 cm

Page 4 of 13 J Nanopart Res (2012) 14:648

123

critical dimension for plasmon resonance. However,

the unchanged value of the absorbance around

400 nm, which is typically used as a measure of the

quantity of metallic gold contained in the colloid

(Kreibig and Vollmer 1995) and which is expected to

decrease progressively during bleaching processes,

means that such post-irradiation does not end up with

particle disintegration and subsequent dissolution. In

contrast, post-irradiation leads to a rearrangement of

particle concentration, average dimension and statis-

tical distribution, which reach a new, highly stable

equilibrium condition. Indeed, Mie-fitting of the

spectra recorded during the bleaching process (Giorg-

etti et al. 2011) shows that, after 800 shots (spectrum

not shown in Fig. 3), the number of particles (N) per

unit volume increased from 1.1 9 1013 up to

6.6 9 1013 N/cm3, while particle average diameter

decreased from 4 down to 2.4 nm, with statistical

distribution characterized by r- = r? = 1 nm. An

analogous behavior is observed with the sample

obtained by 532 nm ablation. The initial spectrum

(thick gray line in Fig. 3) is progressively bleached in

the plasmon region, which is again consistent with an

overall reduction of particle average dimensions.

However, also in this case, no decrease of the

absorbance at 400 nm is observed in the final spectrum

(thin gray line) recorded after 5,200 laser shots,

confirming that no particle dissolution occurs and that

the mechanism at the basis of particle stability in pure

acetone keeps active during green-light post-irradia-

tion. In conclusion, not only green light produces in

Fig. 2 Typical TEM images of some of the suspensions of

AuNPs of Fig. 1: a 1064 nm ablation in pure water; b 532 nm

ablation in pure water; c 1064 nm ablation in pure acetone;

d 532 nm ablation in pure acetone. The insets in a–c represent the

statistical distribution of particle diameters. The inset in

d represents a magnification of the area surrounded by the square

400 500 600 700 800

0.2

0.4

0.6

0.8

1.0

1.2

abso

rban

ce

wavelength (nm)

post-irradiation532 nm 15mJ

pure acetone

Fig. 3 UV–Vis absorption of the samples of Fig. 2c (blacklines) and d (gray lines) before (thick lines) and after (thin lines)

post-irradiation with 532 nm and 15 mJ per pulse

J Nanopart Res (2012) 14:648 Page 5 of 13

123

acetone much smaller particles than IR light, with

narrower statistical distribution, but it does not favor

aggregation nor precipitation of the colloid.

Fluorescence spectroscopy

According to Wilcoxon et al. 1998, the small particle

dimensions of the AuNPs obtained in pure acetone by

532 nm ablation should be accompanied by a consid-

erable fluorescence. To verify that, we carried out

fluorescence spectroscopy tests with different samples

of AuNPs obtained in pure acetone. Some results are

reported in Fig. 4, which compares the excitation and

emission properties of pure acetone (dashed black

lines), of a sample of AuNPs in acetone obtained by IR

ablation (solid black lines) and of a sample of AuNPs

in acetone obtained by 532 nm ablation (solid gray

lines). Only the last sample, that is the one character-

ized by average size far below 5 nm and low

dispersivity, exhibits fluorescence, with excitation

and emission peaks at 400 and 480 nm, respectively.

Furthermore, fluorescence was not detected for sam-

ples prepared in the same ablating conditions in water,

probably due to their larger average size (see Fig. 2).

Phase transfer to water of AuNPs obtained in pure

acetone and subsequent functionalization

with water-soluble compounds

Many applications of our AuNPs in acetone in fields

related to biology or medicine, strongly depend on the

possibility of phase transfer to water, without signif-

icant loss of their properties. In order to assess this

point, we first added ultra pure water to colloidal

suspensions of AuNPs in acetone and let acetone to

evaporate. Then, we checked the presence of acetone

residues by Raman spectroscopy. At the end of the

process, when acetone evaporation was complete, we

recorded UV–Vis and, in the case of samples obtained

by 532 nm ablation, fluorescence spectra of the water-

dissolved AuNPs. Beyond remaining stable over

months, after phase transfer, the gold particles in

water did not exhibit any change of emission or

absorption properties. As an example, a comparison of

the UV–Vis and fluorescence spectra before (black

lines) and after (gray lines) phase transfer from

acetone to water is reported in Fig. 5a, b. In the case

of absorption, the observed difference in the intensity

of the plasmon band is related to dilution.

The second step is particle functionalization. In

order to demonstrate that functionalization is possible,

we used different test compounds, such as adenine and

350 400 450 500 550 600 650

fluo/

exc

(a.u

.)

wavelength (nm)

Fig. 4 Excitation (for emission at 480 nm) and emission (for

excitation at 390 nm) spectra of: pure acetone (dashed blacklines); colloids of AuNPs in pure acetone obtained with

1064 nm, 15 mJ per pulse (solid black lines); colloids of AuNPs

in pure acetone obtained with 532 nm, 15 mJ per pulse (solidgray lines)

abso

rptio

n (a

.u.)

wavelength (nm)400

(a)

(b)

500 600 700 800

450 500 550 600

fluor

esce

nce

(a.u

.)

wavelength (nm)

excitation390 nm

Fig. 5 UV–Vis (a) and fluorescence (b) spectra of AuNPs

obtained by 532 nm, 15 mJ per pulse laser ablation in acetone

before (black lines) and after (gray lines) phase transfer to water.

OPL = 1 cm

Page 6 of 13 J Nanopart Res (2012) 14:648

123

phen. Indeed, adenine is one of the basic components

of RNA and DNA and, consequently, a reference

compounds for the functionalization of medicine-

oriented metallic nanoparticulates (Boisselier and

Astruc 2009; Nam et al. 2004; Elghanian et al.

1997). Phen, which is soluble in different solvents,

including water, can adsorb easily onto metallic

surfaces like Ag, Au, Cu, Ni, Co, Fe (Muniz-Miranda

2000; Zawada and Bukowska 2000; Nikoobakht and

El-Sayed 2003; Peng et al. 2005; Andrade and

Temperini 2007; Muniz-Miranda et al. 2010, 2011;

Sarkar et al. 2010). In this sense, it offers a good test

bed for any study on metal functionalization.

Therefore, we added a water solution of phen to the

Au hydrosol of Fig. 5, in order to have around 10-4 M

concentration and then we checked the functionaliza-

tion by micro-Raman spectroscopy. We deposited a

drop of the colloidal suspension onto a solid substrate

and, after drying, we observed its Raman response

with 785 nm exciting line, whose employment usually

avoids interference of fluorescence. The results are

reported in Fig. 6. Although a significant background

signal is observed, it does not impair the observation

of the Raman bands. In the figure, upper and lower

curves represent the spectra before and after subtrac-

tion of such background, respectively. Since phen is

electronically excited in the UV region, the back-

ground is attributable to the large fluorescence of the

Au NPs, which extends until the NIR region. The

bands closely correspond to the SERS bands previ-

ously observed for adsorption of phen on Au nanorods,

as shown in Table 1, thus confirming that functional-

ization occurred.

We performed analogous functionalization tests

with adenine (Supporting information, Fig. SI1). Also

in this case, the fluorescence background due to

AuNPs obtained with 532 nm is very intense and

almost masks the Raman response of the organic

compound, while it is negligible in the case of the

sample obtained with 1,064 nm, which permits clear

observation of the characteristic bands of adenine

adsorbed onto AuNPs (Kundu et al. 2009).

From these measurements, it is possible to obtain

the indication that AuNPs prepared in acetone and

dispersed in water maintain both fluorescence and

SERS efficiency by excitation with the 785-nm laser

line.

Raman characterization of AuNPs in pure acetone

According to Kawasaki and Nishimura (2006), a

possible stabilization mechanism for AuNPs in acetone

could be related to the interaction of the C=O group with

gold. Formation of graphite or amorphous carbon layers

around their surface is another possibility. For example,

in the case of ns ablation of gold in pure toluene, it was

observed that 1,064 nm pulses caused solvent pyrolysis

and graphite formation (Amendola et al. 2005). Other

authors claim to have observed formation of a graphite

layer surrounding copper NPs produced by laser

ablation in pure acetone (Kazakevich et al. 2006). To

clarify this point, we performed Raman and micro-

Raman measurements on our samples.

Fig. 6 Micro-Raman spectrum of phen-functionalized AuNPs

before (upper curve) and after (lower curve) background

subtraction. Excitation line: 785 nm

Table 1 Comparison among Raman tests performed on phen

powders and on phen molecules after adsorption onto differ-

ently shaped Au surfaces

Vibration Raman

powder

sample

(Muniz-

Miranda 2000)

SERS Au

nanorods

(Nikoobakht and

El-Sayed 2003)

SERS Au

nanoparticles

(present

study)

m2 (A1) 411 420 420

m3 (A1) 552 555 553

m4 (A1) 711 721 720

m6 (A1) 1035 1051 1049

m12 (A1) 1294 1300 1300

m13 (A1) 1345 1342 1340

m14 (A1) 1405 1407 1407

m15 (A1) 1446 1448 1447

m16 (A1) 1502 1512 1510

J Nanopart Res (2012) 14:648 Page 7 of 13

123

Figure 7 compares the Raman spectrum of pure

acetone with that of the colloid of Fig. 2d, which

exhibits a weak band at 1,560 cm-1, not ascribed to

acetone vibrations. The detection of such band is

justified by the SERS effect, due to the resonance of

the laser exciting line (514.5 nm) with the plasmon

resonance band of the AuNPs (see Fig. 1). Similar

results (not shown) were obtained with the colloid of

Fig. 2c.

The band at 1,560 cm-1 can be attributed to the

formation of enolate ions, which can adsorb on the

surface of the positively charged gold particles,

according to the schema reported in Fig. 8a. Actually,

enolate cannot be present as free anion in acetone, but

only adsorbed on metal. Acetone enolate is character-

ized by two resonant forms, where C=O or C=C bond

(not present in pure acetone) occur (Fig. 8b). Typi-

cally, the alkenic C=C stretching modes should occur

in the 1,600–1,650 cm-1 spectral region. However, in

the case of enolate, the electronic conjugation with the

nearby C=O bond should move the band to lower

frequency, making it compatible with the observed

band at 1,560 cm-1. This latter, in conclusion, can be

confidently attributed to a mixing of C=O and C=C

stretching modes.

This assignment agrees with previous conclusions

reported in the literature. The existence of surface-

bound acetone enolate was postulated on the basis of

infrared spectroscopic studies of acetone adsorption

on various metal oxides. Absorption bands reported at

1,558, 1,540, and 1,543–1,595 cm-1 have been

assigned to m(CO) of acetone enolate on NiO (Miyata

et al. 1974), Fe2O3 (Busca and Lorenzelli 1982) and

Al2O3 (Hanson et al. 1987; Zaki et al. 2000).

Organometallic complexes of Ru and Pd containing

acetone enolate are also known, with characteristic

frequencies observed at 1,579 cm-1 (Hartwig et al.

1991) and at 1,554–1,565 cm-1 (Vicente et al. 1998),

respectively. On the other hand, for acetone adsorbed

on Pt (Vannice et al. 1991), vibrational bands in the

1,530–1,585 cm-1 spectral region were detected by

RAIRS and HREELS experiments and attributed to

acetone enolate, as successive DFT studies confirmed

(Jeffery et al. 2005). Finally, acetone enolate bound to

Ni surface was attributed by DFT studies to the species

showing a RAIRS band at 1,545 cm-1 (Sim et al.

2002).

To have a deeper insight into the nature of the

observed band, we also performed micro-Raman

measurements by drying relevant amounts of the same

colloid used for Fig. 7 in a steel spoon and adopting

long accumulation times (10 9 200 s). The resulting

spectrum is reported in Fig. 9. Two broad bands at

about 1,590 and 1,350 cm-1 are assigned as G and D

bands of graphitic carbon, whereas that occurring at

1,555 cm-1 corresponds to the C=O/C=C stretching

mode of acetone enolate observed in the acetone

suspension (see Fig. 7). The bands observed between

1,200 and 1,500 cm-1 are, instead, attributable to

those deriving from amorphous carbon (sp2 or sp3

type), intensified by SERS effect, as widely reported in

Fig. 7 Raman spectrum of pure acetone (lower curve) and a

colloid of AuNPs obtained in pure acetone by 532 nm, 15 mJ

per pulse laser ablation (upper curve). Exciting line: 514.5 nm

Fig. 8 a Adsorption of acetone on a gold cluster; b resonant

forms of acetone enolate

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the literature (Hu et al. 2009; Perevedentseva et al.

2006; Rusciano et al. 2008; Veres et al. 2004). In

contrast, no such bands were observed in colloids

prepared by 1,064 nm ablation.

Therefore, we can deduce that Au colloids pro-

duced in acetone by 532 nm ablation contain a

sizeable amount of carbon, which could be formed

by photo degradation of acetone enolate adsorbed on

metal during pulsed laser irradiation. Actually, when

the same irradiation regime is used in acetone without

gold target, no evidence of carbon results from both

UV–Vis absorption and Raman scattering.

f-potential measurements

The stability of metal particles in colloidal suspen-

sions depends on the presence of surface electric

charges. In the case of gold particles obtained by laser

ablation in acetone, the presence of acetone enolate,

ascertained by Raman spectroscopy, could be deter-

minant for the stability of the colloids. In this sense, f-

potential measurements can provide useful informa-

tion. Actually, the f-potential represents the electric

potential difference occurring between the charges of

the species strongly adsorbed on the metal surface and

those (with opposite sign) of the diffuse layer in the

dispersing medium.

We measured the f-potential of the colloids

obtained in acetone after phase transfer to pure water.

The results are reported in Table 2. The negative sign

of the f-potential is to be attributed to adsorbed

negative ions like enolates or hydroxide anions

deriving from the aqueous medium. The large value

obtained in the case of 1,064 nm ablation fully justifies

the extraordinary long-term stability. In contrast, in

the case of samples obtained by 532 nm ablation, the

lower value of the f-potential, which is not accompa-

nied by a reduced stability, suggests that carbon could

also play a role in the stabilization process.

Discussion

In the case of the AuNPs obtained in pure acetone by

532 nm ablation, the negative value of the f-potential,

as well as the Raman results, point to a coexistence of

both carbon and enolate on the surface. We propose

that, upon laser irradiation, enolate forms in the

solution according to the mechanism described in

Fig. 8 and adsorbs onto the gold NPs, promoting their

long-term stability. Further interaction with laser

pulses can cause degradation of enolate to amorphous

carbon, which is favored under green-light irradiation

and for those molecules which, being in the vicinity of

particles surface, experience plasmon-enhanced local-

ization of the impinging electromagnetic field. As a

matter of facts, we did not observe any Raman

evidence of carbon formation on samples obtained by

1064 nm ablation.

Figure 3 shows that 532-nm post-irradiation of

AuNPs in pure acetone causes particle rearrangement,

but it does not end up with thorough bleaching and

dissolution of gold. Figure 10 reports the effect of the

same type of post-irradiation on the same samples

used for the tests of Fig. 3, but after their phase

transfer to water. Differently from the case of Fig. 3,

Fig. 10a, b shows that, in this case, progressive

particle dissolution occurs, as evidenced by the

decrease of the absorbance at 400 nm. Such decrease

accompanies the bleaching of the plasmon peak,

indicating that fragmentation by-products cannot

undergo the same solvent-mediated stabilization

mechanism, which is active in acetone. It confirms

Fig. 9 Micro-Raman spectrum of a colloid of AuNPs obtained

by 532 nm laser ablation in pure acetone, after drying onto a

solid substrate. Exciting line: 514.5 nm

Table 2 f-potential tests

Preparation

wavelength

(nm)

Electrophoretic

mobility

(lm cm/V s)

f-potential

(mV)

Width

(mV)

1064 3.5 ± 0.9 -45 12

532 2.7 ± 0.6 -35 7

J Nanopart Res (2012) 14:648 Page 9 of 13

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that the stabilization mechanism of AuNPs in pure

acetone is strictly related to laser-solvent interaction.

Emission from small gold NPs has been observed

previously by several authors (Giorgetti et al. 2009,

2011; Wilcoxon et al. 1998; Zheng et al. 2003; Tran

et al. 2006; Zheng et al. 2004). However, the reliability

of such studies is often impaired by the difficulty in

separating the contribution of the capping agents from

the overall emission of the colloid. For example, in the

case of highly fragmented PAMAMG5-capped AuN-

Ps, the emission was rather to be attributed to metal-

enhanced fluorescence of PAMAMG5 than to gold

nano- or subnano-clusters (Giorgetti et al. 2011). In

the case of AuNPs obtained in pure acetone by 532 nm

ablation, no stabilizing molecule was intentionally

added to the colloid. In this sense, such systems are the

closest ones to perfectly uncapped AuNPs. Although

we could easily exclude any contribution of pure

acetone by checking solvent fluorescence before and

after laser irradiation, this was not the case for the

products of light-induced degradation of acetone at

particles surface, like acetone enolate or carbon. These

latter species could also provide some contribution to

the fluorescence emission which, however, can hardly

be isolated and evaluated separately from that of gold.

For example, some species of amorphous carbon emit

at 468 nm albeit excited at the shorter wavelength of

193 nm (Lormes et al. 1998; Damm et al. 2001;

Rholfing 1988). However, a more in-depth analysis

devoted to disentangle the role of carbon, if any, was

beyond the scope of this work.

Conclusions

We prepared AnNPs in pure acetone by ps laser

ablation at 1,064 and 532 nm. Green-light ablation

permits production of very small particles, with

narrow statistical distribution. Moreover, the extre-

mely small average size (2–3 nm) makes them fluo-

rescent. This property is not detected in AuNPs

prepared in pure water, which result bigger. In general,

the particles are characterized by an extraordinary

long-term stability, which we attribute to formation of

acetone enolate on particle surface, which gives them

a negative repulsive charge. Differently from the water

case, such stabilization mechanism impairs thorough

bleaching of the suspensions under ps post-irradiation

and increases colloid resistance to 532-nm short

pulses. Indeed, the effect of post-irradiation is limited

to a rearrangement of particle average size and

statistical distribution, with no precipitation or disso-

lution of gold.

Due to their extraordinary stability, AuNPs

obtained in pure acetone can be stored on-shelf for

years, transferred to the water phase and subsequently

functionalized with different organic compounds on

demand. The phase transfer maintains stability and,

when present, fluorescence. Moreover, after phase

transfer, the NPs still exhibit SERS activity, which we

tested by near IR (785 nm) excitation, that is the most

convenient one in biological applications, since it

avoids any interference deriving from the biological

environment. As a whole, our systems exhibit prom-

ising characteristics for the development of biocom-

patible fluorescent or Raman markers to be used in in

vitro, as well as in in vivo detection of molecule/

particle nanohybrids.

0.5

1.0

1.5(a)

(b)

post-irradiation

532 nm 15mJ

abso

rban

ce

wavelength (nm)

water phase

400 500 600 700 800

400 600 800

0.4

0.8

1.2 post-irradiation

532 nm 15mJ

abso

rban

ce

wavelength (nm)

water phase

Fig. 10 Post-irradiation with 532 nm, 15 mJ per pulse of the

samples of Figs. 2c (a) and d (b), after phase transfer to water.

Thick black lines initial spectra; thick gray lines final spectra

obtained after 2,400 (a) and 5,200 (b) shots. Intermediate thinlines, from top to bottom: spectra recorded for increasing

number of shots

Page 10 of 13 J Nanopart Res (2012) 14:648

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Acknowledgments Funding from the project NABLA (Decree

n.4508—September 1, 2010 by Regione Toscana, Italy, PAR FAS

2007–2013 funds, Action 1.1.a.3) and project PRIN2009 Novelplasmon-based processes and materials for sensor applications of

the Italian Ministry of Research is acknowledged. The authors

also wish to thank Gabriella Caminati (Chemistry Department of

the University of Firenze, Italy) and Alessio Rindi (Department

of Chemistry and Industrial Chemistry of the University of

Genova, Italy) for technical assistance with f-potential and TEM

characterizations, respectively.

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