Self-assembly of monolayer-coated silver nanoparticles on gold electrodes. An electrochemical investigation

Article abstract of DOI:10.1135/cccc20031395

Redox behaviour of an array of dodecanethiol-coated silver nanoparticles, self-assembled on the gold electrode surface by dithiol linkers, is dominated in aqueous electrolytes by a reversible faradaic process that has been attributed to the anodic oxidation of silver nanoparticles. The nanoparticle array may be switched between the oxidised and reduced states repeatedly without any significant loss of electroactive material, thus showing a remarkable stability under electrochemical conditions. The mainly capacitive high-frequency electrical response of the nanoparticle array/electrolyte interface is characterised by a low value of capacitance, typical of a self assembled monolayer (SAM) of long-chain alkanethiols on gold; it has been associated with the compact organic layer interposed between the nanoparticles and the gold substrate. At lower frequencies, the interface displays a poorer electrical behaviour with both capacitive and resistive elements, which was associated with the more disordered organic layer located on the outer side of the film.

Full text of DOI:10.1135/cccc20031395

Monolayer-Coated Silver Nanoparticles
1395
SELF-ASSEMBLY OF MONOLAYER-COATED SILVER NANOPARTICLES
ON GOLD ELECTRODES. AN ELECTROCHEMICAL INVESTIGATION
1
2
3
Massimo MARCACCIO , Massimo MARGOTTI , Marco MONTALTI ,
4
,
5
6,
Francesco PAOLUCCI *, Luca PRODI and Nelsi ZACCHERONI *
Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2,
1
2
4
0126 Bologna, Italy; e-mail: maxmar@ciam.unibo.it, massimomargotti@ciam.unibo.it,
3
4
5
6
mmont@ciam.unibo.it, paolucci@ciam.unibo.it, lprodi@ciam.unibo.it, nelsiz@ciam.unibo.it
Received May 27, 2003
Accepted July 9, 2003
This paper is dedicated to Professor Sergio Roffia, an esteemed scientist and a great teacher.
Redox beh aviour of an array of dodecan eth iol-coated silver n an oparticles, self-assem bled on
th e gold electrode surface by dith iol lin kers, is dom in ated in aqueous electrolytes by a re-
versible faradaic process th at h as been attributed to th e an odic oxidation of silver
n an oparticles. Th e n an oparticle array m ay be switch ed between th e oxidised an d reduced
states repeatedly with out an y sign ifican t loss of electroactive m aterial, th us sh owin g a re-
m arkable stability un der electroch em ical con dition s. Th e m ain ly capacitive h igh -frequen cy
electrical respon se of th e n an oparticle array/electrolyte in terface is ch aracterised by a low
value of capacitan ce, typical of a self assem bled m on olayer (SAM) of lon g-ch ain alkan eth iols
on gold; it h as been associated with th e com pact organ ic layer in terposed between th e
n an oparticles an d th e gold substrate. At lower frequen cies, th e in terface displays a poorer
electrical beh aviour with both capacitive an d resistive elem en ts, wh ich was associated with
th e m ore disordered organ ic layer located on th e outer side of th e film .
Keyw ord s: Silver n an oparticles; Electroch em istry; Cyclic voltam m etry; Electroch em ical im -
pedan ce spectroscopy; Self assem bled m on olayers.
Th e wide in terest in th e recen t years in n an oscale en gin eerin g h as been tes-
tified by th e h uge an d perm an en tly in creasin g am oun t of available litera-
ture on th is topic, wh ich describes m an y attem pts to realise devices in th e
1
,2
n an oscale ran ge capable of perform in g useful fun ction s , such as m in ia-
3
,4
5,6
turised sen sin g un its an d data processin g elem en ts for various applica-
7
tion s . Facin g th e problem from a ch em ical poin t of view, th ere is a n eed
for system s extrem ely well ch aracterised at th e m olecular level, sin ce deal-
in g with n an oscale m aterials m ean s explorin g properties derived from
quan tum -level ph en om en a an d n ot (or n ot on ly) from m acroscopic proper-
Collect. Czech. Chem. Commun. (Vol. 68) (2003)
doi:10.1135/cccc20031395

1
396
Marcaccio et al.:
ties. In th is con text, n an oparticles are attractin g m uch in terest sin ce th eir
8
properties can be tun ed on ch an gin g th eir dim en sion s an d/or th e species
lin ked on th eir surface, im partin g th em extrem ely varied ch em ical, electri-
cal an d optical ch aracteristics9 . In particular, self-assem bly of m etal
–11
1
2
n an oclusters on a variety of substrates, form in g eith er 2D or 3D structures
1
3
an d superstructures , h as been widely in vestigated over a n um ber of
1
2–14
years
. Various strategies for th e fabrication of such system s h ave been
reported wh ich gen erally exploit bifun ction al ligan ds to accom plish eith er
th e in terparticle lin kin g an d/or th eir an ch orin g on to th e substrate sur-
1
2–14
face
. Nan oparticles arrays of eith er m etallic or sem icon ductin g m ateri-
als, organ ised in to con trolled arch itectures on relevan t electron ic sub-
strates, h ave been , for in stan ce, in vestigated for th e fabrication of n ew
1
4
gen eration s of electron ic devices an d for sen sor application s . Electro-
ch em istry h as provided useful tools for th e ch aracterisation of such sys-
1
2e,12h ,12j,12k,13c,15,16
tem s
, an d, in particular, m ost of th e electroch em ical in -
vestigation s on m on olayer-protected n an oparticles h ave focused on th e pe-
culiar, size-depen den t electrical properties of th e n an oparticles/electrolyte
1
5
in terface . Surface assem blies of gold or silver n an oparticles on gold or
Al O electrodes display in fact (sub)attofarad m olecular capacitor ch arac-
2
3
1
5
16
teristics , sim ilar to th ose of solution s of n on -an ch ored n an oparticles . By
con trast, oth er im portan t aspects of th e electroch em ical beh aviour of th ese
system s, such as th e stability of th e organ ised assem bly on electroch em ical
m odification s of th e n an oparticle surface, e.g., th e an odic growth of oxide
film s, h ave received relatively less atten tion 14
.
We would like th erefore to presen t h ere a detailed electroch em ical study
of m on olayer-coated silver n an oparticles covalen tly lin ked to a gold elec-
trode. Besides its im plication in electron ic devices, such system m ay also be
poten tially in terestin g, due to th e reported biological activity of silver/silver
1
7
oxide n an oparticles , for th e realisation of an tim icrobial coatin gs in bio-
m edical devices. Th e strategy th at we h ave followed for th e con struction of
th e n an oparticle array, takes advan tage from th e great affin ity th at gold
an d silver h ave for th e th iol fun ction . We h ave syn th esised silver n an o-
particles bearin g dodecan e-1,12-dith iol on th e surface, on e of th e term in al
SH fun ction bein g lin ked to th e silver surface an d th e oth er, to be lin ked to
th e gold electrode.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Monolayer-Coated Silver Nanoparticles
1397
EXPERIMENTAL
Materials
Decan e-10-th iol (1), all salts an d solven ts were obtain ed from Aldrich an d used as supplied.
Water of MilliQ grade was used.
Syn th esis of Dodecan e-1,12-dith iol (2)
1
,12-Dibrom ododecan e (6.0 m m ol) an d th iourea (7.8 m m ol) were refluxed in water (15 m l)
for 20 h . Next, 1.5 M sodium h ydroxide (10 m l) was added an d th e reflux con tin ued for an -
oth er 6 h . A wh ite precipitate appeared on coolin g. It was filtered off after h avin g set pH to
1
.0 by addin g a con cen trated solution of sulfuric acid an d copiously wash ed with water to
1
afford 0.42 m m ol of pure product. H NMR (300 MHz, CDCl ; δ, ppm ): 2.5 (q, 4 H, CH SH),
3
2
1
.6 (quin tet, 4 H, CH CH SH), 1.3 (m , 16 H, (CH ) CH CH SH).
2
2
2
8
2
2
Preparation of Modified Silver Nan ocrystals
Ag-(1+2) n an oparticles were prepared as already reported18, usin g a solution of cappin g
agen ts at a 9:1 m olar ratio of 1 an d 2. Th e n an ocrystals form ed were precipitated by addi-
tion of eth an ol, filtered off an d wash ed with eth an ol to obtain a dry gray powder
redispersible in ch loroform . Th e m odified n an oparticles were used with out an y furth er puri-
fication . Tran sm ission electron m icrograph s (TEMs) were obtain ed usin g a Ph ilips CM 100
tran sm ission electron m icroscope (Fig. 1).
1
00
8
6
0
0
4
2
0
0
d = 3.4 nm σ = 0.5 nm
0
2
4
6
8
10
12
d, nm
FIG. 1
Particle size h istogram s of as-produced silver n an oparticles by statistical an alysis. In set: tran s-
m ission electron m icroscopy m icrograph of as-prepared silver n an oparticles. For TEM in vesti-
gation s, a drop of n an oparticle solution in CH Cl was tran sferred on to h oley carbon foils
2
2
supported on con ven tion al copper m icrogrids. A Ph ilips CM 100 tran sm ission electron m icro-
scope, operatin g at 80 kV, was used
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

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Marcaccio et al.:
Electrode Modification
Th e electrodes used for n an oparticle im m obilisation were 200 n m th ick gold film s evaporated
–
2
in h igh vacuum on con ductive fluorin e-doped Sn O2 glass (sh eet resistan ce of 8–10 Ω cm ).
After im m ersion in a dilute solution of silver colloid in dich lorom eth an e, th e m odified gold
electrode was th orough ly rin sed with dich lorom eth an e an d dried before use.
Electroch em ical In strum en tation an d Measurem en ts
Electroch em ical experim en ts were perform ed in 1 × 10–1 M Na SO aqueous solution s (pH
2
4
6
.5), usin g a two-com partm en t electroch em ical cell equipped with a saturated calom el elec-
trode (SCE) an d a platin um spiral as coun ter electrode. Cyclic voltam m etric (CV), ch ron o-
am perom etric an d electroch em ical im pedan ce spectroscopy (EIS) experim en ts were carried
out with an Autolab Model PGSTAT 30.
RESULTS AND DISCUSSION
Cyclic Voltammetry
Th e redox properties of th e silver n an oparticle-m odified gold electrodes
were in vestigated by cyclic voltam m etry an d ch ron oam perom etric experi-
m en ts, wh ich also allowed to estim ate th e surface coverage of th e silver
colloid. After m odification , th e gold electrode was tran sferred in to th e two-
–
1
com partm en t electroch em ical cell, con tain in g an aqueous 1 × 10 M Na SO
2
4
solution . Figure 2a sh ows th e CV curve obtain ed by perform in g repetitive
poten tial scan s un der th e above con dition s. Th e first scan , from n egative to
positive poten tials, displays a very broad oxidation peak in th e region cen -
tred at 0.48 V, labelled as A′, an d in th e reverse scan a m uch n arrower re-
duction peak at 0.10 V, labelled as B′. In th e secon d scan , th e h eigh t of th e
broad an odic peak is greatly dim in ish ed wh ile a n arrow peak appears at
0
.22 V (A) wh ose h eigh t in creases progressively in th e followin g scan s. At th e
sam e tim e, a n ew cath odic peak (B) in creases at 0.07 V at th e expen se of th at
at 0.10 V. A sim ilar beh aviour was also foun d in aqueous LiClO solution s.
4
Th e presen ce of isopoten tial poin ts in th e CV curves (at 0.09 an d 0.24 V),
associated with th e con version between two electroactive system s sh owin g
1
9
sligh tly differen t redox properties , would suggest th at, durin g th e first few
poten tial scan s, a sign ifican t reorgan isation takes place in th e silver colloid
layer. Such a con version would occur on th e voltam m etric tim e scale with -
out an y loss of electroactive m aterial, as th e overall ch arge un dern eath th e
two eith er an odic (A, A′) or cath odic (B, B′) peaks rem ain s con stan t. Th e
lim itin g CV pattern , obtain ed after ca ten scan s, is sh own in Fig. 2b: th e
peak m orph ology is th at typical of redox processes in volvin g surface-
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Monolayer-Coated Silver Nanoparticles
1399
2
0
con fin ed species . As expected for such a system , th e peaks h eigh t was
2
0
foun d to in crease lin early with scan rate . Furth erm ore, in tegration of th e
–
1
CV curve sh owed th at, at an y scan rate from 0.01 to 0.5 V s , n early th e
sam e ch arge is exch an ged in th e an odic an d cath odic peaks (55.4 vs
–
2
5
5.0 µC cm , respectively), as also con firm ed by ch ron oam perom etric ex-
perim en ts. Th e two peaks in Fig. 2b are th erefore coupled to each oth er as
partn ers of a reversible redox couple con fin ed at th e surface of th e gold
electrode. In agreem en t with such a h ypoth esis, Fig. 3 sh ows th at reduction
process B is on ly observed wh en th e electrode first un dergoes th e oxidation
process A, an d vice versa. It also in dicates th at, on th e tim e scale of th e CV
experim en t, both processes are quan titative.
Such a reversible redox process is clearly associated with th e presen ce of
th e silver colloid on th e gold surface. Un der th e em ployed experim en tal
con dition s, in fact, th e un m odified gold electrode on ly displays a purely ca-
pacitive beh aviour, ch aracterised by a m uch larger capacitive curren t th an
in th e m odified electrode (dash ed lin e in Fig. 2b). Th e low double-layer ca-
pacitan ce observed in th e latter case, on th e oth er h an d, is typical of elec-
2
1
trodes carryin g self-assem bled m on olayers (SAM) of lon g-ch ain th iols .
Th e CV respon se of th e gold substrate supportin g a SAM of dodecan eth iols
is com pared in Fig. 2b (dot-an d-dash ) with th e precedin g on es, displayin g
in fact a greatly dim in ish ed capacitive curren t com parable to th at of n an o-
8
0
100
50
0
A
a
b
4
0
A′
0
–
50
–
40
B
B′
0
.0
0.4
0.8
–0.2
0.0
0.2
0.4
E, V vs SCE
E, V vs SCE
FIG. 2
a Con secutive cyclic voltam m etric curves of as-prepared Ag colloid film on Au electrode in an
–
1
aqueous 1 × 10 M Na SO solution . Th e dash ed lin e correspon ds to th e first scan ; b full lin e:
2
4
as in a after ca ten scan s, dash ed lin e: clean Au surface, dot-an d-dash ed lin e: Au m odified by a
–
1
dodecan eth iol SAM. Scan rate: 0.05 V s , T = 25 °C
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

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400
Marcaccio et al.:
particle-m odified electrodes. Th e oxidation an d reduction peaks in Figs 2
an d 3 are th erefore attributed to a process in volvin g silver n an oparticles
an d, in particular, to th e form ation of a silver oxide or h ydroxide layer over
th e colloid surface, durin g th e positive scan , an d to th e correspon din g
reductive strippin g, durin g th e reverse scan . At th e operatin g pH, th e for-
m ation of an (h ydr)oxide silver layer is in fact th erm odyn am ically allowed
2
2
at poten tials h igh er th an 0.1 V (vs SCE) . Such a h ypoth esis was con firm ed
by com parison with a m assive silver electrode m odified with a SAM of
dodecan eth iols, displayin g a very sim ilar CV beh aviour. Furth erm ore, a si-
m ilar CV beh aviour, also attributed to th e form ation of a den sely packed
m on olayer of h ydroxide species, h as been reported for colloidal gold
1
3,14
n an oparticles con fin ed on ITO via an (am in opropyl)siloxan e film
.
In order to estim ate th e surface den sity of silver n an oparticles on th e
gold electrode surface, th e ch arge exch an ged at th e CV peaks (Fig. 2) was
con sidered. By com bin in g such a value with (i) th e m on olayer capacity of
–
2 23
silver (280 µC cm ) , (ii) th e diam eter of silver particles as obtain ed by th e
TEM m easurem en ts, an d (iii) assum in g th at th e en tire surface of each
n an oparticle un dergoes th e redox process, th e surface den sity of th e self-
1
0
–2
assem bled colloid m on olayer was estim ated to be 9.7 × 10 particle cm
.
Such a low value would correspon d to a rath er loose packin g of th e silver
n an oparticles on th e gold surface, th e upper lim it, correspon din g to th e
1
2
–2
den se packin g as sh own from th e TEM im age, bein g 4 × 10 particle cm
.
A
2
0
0
–
20
B
–
0.2
0.0
0.2
0.4
E, V vs SCE
FIG. 3
Two con secutive CV curves of Ag colloid film on to Au electrode in an aqueous 1 × 10
Na SO solution , after in itial con dition in g as sh own in Fig. 2a. Poten tial sequen ces: first scan
–
1
M
2
4
–
1
(
full lin e) +0.15/–0.18/+0.3 V, secon d scan (dash ed lin e) +0.15/+0.3/–0.18 V. Scan rate: 0.1 V s
,
T = 25 °C
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Monolayer-Coated Silver Nanoparticles
1401
Th e CV experim en ts would th erefore in dicate th at on ly 5% of th e gold sur-
face is covered. However, th e very low capacitive curren t m easured at th e
m odified electrodes, an d th e results of th e electroch em ical im pedan ce spec-
troscopic in vestigation (vide infra), suggest th at such a coverage is un deres-
tim ated, likely due to th e fact th at th e silver surface actually in volved in
th e redox process is on ly a fraction of th e wh ole n an oparticle surface. Th is
would n ot be surprisin g in view of th e well-kn own h igh ly blockin g proper-
2
1
ties of SAMs of lon g-ch ain th iols on coin m etals (vide infra) .
Electrochemical Impedance Spectroscopy
EIS in vestigation of th e m odified electrode/solution in terface was carried
out un der th e sam e con dition s used in th e CV an d ch ron oam perom etric
experim en ts, sin ce EIS allows a m uch closer in spection of th e electrical
structure of th e in terface with respect to capacity m easurem en ts based on
2
4
CV or ch ron oam perom etry . EIS h as been largely used for in vestigatin g th e
2
1–25
kin etics of redox processes occurrin g at n-alkan eth iol-coated electrodes
,
an d was also used to study th e electroch em ical properties of m etal n an o-
particle colloids, eith er in solution or im m obilised on con ductive sup-
1
5,16
ports
. In EIS, th e curren t is m easured un der poten tiostatic con trol, i.e.
1
6
a
b
2
0
1
2
1
1
5
0
5
0
3
16 mHz
8
4
0
0
.63 kHz
0
.20 kHz
6
1
9.95 Hz
8
0
4
12
0
2
4
8
Re(Z), kΩ
Re(C), µF
FIG. 4
Com plex plan e plots of th e im pedan ce (a) an d com plex capacitan ce (b) of Ag colloid film on
–
1
Au electrode in an aqueous 1 × 10 M Na SO solution , after in itial con dition in g as sh own in
2
4
Fig. 2a. Th e frequen cy ran ge was 0.1 Hz–100 kHz. Applied poten tials (in V): –0.30 (●), –0.05
●), 0.20 (ᮀ), 0.45 (●), 0.70 (∆)
(
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1
402
Marcaccio et al.:
at a con stan t bias, wh ile a sm all-am plitude sin usoidal voltage sign al is
2
4
applied to th e electroch em ical cell . Excitation frequen cies (f) between
m Hz an d 1 MHz can be used, th ereby perm ittin g in prin ciple th e in vesti-
1
gation of ph en om en a on tim e scales ran gin g over 9 decades, wh ich m ay
be resolved in th e EIS spectra accordin g to th eir relaxation tim e. In Fig. 4,
th e im pedan ce data recorded at various biases, from n egative (–0.3 V) to
positive values (+0.7 V), i.e. across th e region wh ere oxidation of n an o-
particles occurs, are sh own .
Two altern ative presen tation s of th e data sets were used. In Fig. 4a, th e
out-of-ph ase com pon en t of th e im pedan ce, –Im (Z), is plotted vs th e in -
ph ase on e, Re(Z), –Im (Z) an d Re(Z) bein g param etric fun ction s of th e fre-
quen cy (z-plot or Nyquist plot)2
0b,24
. Such a presen tation em ph asises th e
in terface respon se at relatively low frequen cies. In Fig. 4b, th e com plex ca-
pacitan ce C is plotted (c-plot), wh ere C = 1/jωZ, j = −1 an d ω th e an gular
2
7
frequen cy (= 2πf) . Th e c-plot em ph asises th e in terface respon se at h igh er
frequen cies with respect to th e z-plot2
5e,26
. Th e electrical respon se of th e
(
m odified) electrode/electrolyte in terface at various frequen cies of th e sin u-
soidal perturbation is usually described in term s of equivalen t circuits 0b,24,
such as th at sh own in Sch em e 1, wh ich reproduces th e beh aviour of th e
presen t system .
2
Th e EIS data sh own in Fig. 4b eviden ce th e blockin g (capacitive) beh av-
iour (series RC arran gem en t) ch aracterisin g th e in terface at h igh (f > 100 Hz)
frequen cy. Th e respon se at low frequen cy (Fig. 4a) reflects, in stead, a poorly
defin ed structure wh ose electrical beh aviour is represen ted in th e equiva-
len t circuit of Sch em e 1 by resistan ce R an d capacitan ce C . Th e R an d C
x
x
elem en ts in th e equivalen t circuit of Sch em e 1 were evaluated by fittin g
2
8
procedures, usin g th e CNLS m eth od described by Boukam p . In th e circuit
of Sch em e 1, R represen ts th e solution resistan ce, C th e double layer ca-
Ω
dl
pacitan ce, wh ile R an d C are elem en ts respon sible for th e in terface beh av-
x
x
2
9
iour at f < 1 Hz . Th e best-fit values of th e various elem en ts in th e circuit
of Sch em e 1 are reported as a fun ction of poten tial in Table I.
C
dl
R
Ω
C
x
R
x
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Monolayer-Coated Silver Nanoparticles
1403
As expected, th e value of R (th e solution resistan ce) is essen tially con -
Ω
stan t at th e various poten tials sin ce such an elem en t is n ot related to elec-
troch em ical processes occurrin g at th e in terface. Also Cdl was foun d to be
alm ost in varian t with poten tial. By con trast, R ch an ges with poten tial.
x
However, its value at 0.7 V is on ly twice as h igh as th at at –0.3 V; such an
elem en t can n ot th erefore be associated to a faradaic process occurrin g at th e
in terface. Th e observed depen den ce of both R an d C on poten tial is possi-
x
x
bly a con sequen ce of th e oxide-layer form ation on th e n an oparticle surface.
Th e low value for double layer capacitan ce Cdl is in agreem en t with th e
low capacitive curren ts m easured by CV an d is com patible with th e pres-
en ce of a rath er com pact organ ic layer on th e gold electrode. In th e case of
a gold substrate m odified with a SAM of dodecan eth iol, th e correspon din g
value was in fact ca 3.0 µF cm –2. Th e very sim ilar Cdl values obtain ed for th e
two system s would th erefore suggest th at th e m on olayer-coated silver
TABLE I
Calculated param eters from com puter fittin g (n orm alised to th e electrode geom etric area),
usin g th e equivalen t circuit in Sch em e 1 for th e Ag colloid film on Au in an aqueous 1 ×
–
1
1
0
M Na SO solution (T = 25 °C)
2 4
RΩ, Ω cm 2
Cdl, µF cm –2
Rx, kΩ cm 2
Y × 10 , Ω cm sϕ (ϕ)a
6
2
E, V
0
–
–
0.30
0.05
88
89
88
88
88
3.3
3.4
3.3
3.8
3.8
1.50
1.55
2.09
2.94
3.02
1.6 (0.7)
1.7 (0.7)
2.0 (0.7)
1.0 (0.7)
1.0 (0.7)
0
0
0
.20
.45
.70
a
See n ote29
.
R
Ω
C
x
C
dl
R
x
FIG. 5
Sch em atic represen tation an d correspon din g equivalen t circuit of self-assem bly of surface-
active m on olayer-coated Ag n an oparticles on Au electrode
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Marcaccio et al.:
n an oparticles are able to form a very com pact film on th e gold electrode,
at varian ce with th e low coverage value estim ated from th e CV ch argin g ex-
perim en ts.
Th e structure of th e silver n an oparticles film , as em ergin g from th e an aly-
sis of th e EIS data, ch aracterised by a com pact in n er layer (h avin g low ca-
pacitan ce) an d a disordered outer on e, m ay in prin ciple explain th e low
ch arges associated with th e oxide-layer form ation , as deduced from th e CV
an d ch ron oam perom etric experim en ts. Th e oxidation process would in fact
in volve prin cipally th e fraction (<<1) of th e n an oparticle surface th at faces
th e aqueous solution an d is free of th iols, bein g th erefore in direct con tact
with th e electrolyte (Fig. 5). Fin ally, th e occurren ce of th e large-am plitude
faradaic processes associated with th e an odic growth /cath odic dissolution
of silver oxide on /from th e n an oparticle surface, an d th e relatively large
double-layer capacitan ce displayed by th e n an oparticle film , would explain
wh y quan tised capacitan ce ch argin g ph en om en a1
presen t system .
5,16
are n ot observed in th e
CONCLUSIONS
Self-assem blin g of m on olayer-protected silver n an oparticles on to a gold
electrode was obtain ed by usin g dith iol lin kers. Th e n an oparticles were
electroch em ically surface-active, un dergoin g, in aqueous electrolyte, a re-
versible an odic oxidation process. Rem arkably, th e assem bly rem ain s stable
over repetitive oxidation /reduction of th e n an oparticle surface. Electro-
ch em ical im pedan ce spectroscopy was used in order to ch aracterise th e
electroch em ical in terface: all th e im pedan ce spectra, obtain ed at various
poten tials in th e region of surface electroactivity of n an oparticles, were
sim ulated by a relatively sim ple equivalen t circuit in wh ich two differen t
capacitive elem en ts accoun t for th e respon se, at h igh an d low frequen cy, of
th e in terface. Such elem en ts were associated, respectively, to an in n er an d
com pact alkan eth iol layer in terposed between th e gold surface an d th e
n an oparticles, an d an outer an d relatively looser alkan eth iol layer located
outside. Th e latter layer would allow easy perm eation of electrolyte to large
fraction s of th e silver surface th us explain in g th e observed CV beh aviour.
This work was carried out with partial support by the University of Bologna (“Funds for Selected Re-
search Topics”), MIUR (PRIN 2002, prot. 2002035735), and C.N.R. (Agenzia 2000). We would like
to thank Dr G. Falini for his help in the TEM investigation.
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5b,25e
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2
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