Metallisation of gel surfaces under ambient conditions

Article abstract of DOI:10.1039/c1cc14472c

A room temperature method to coat a non-conducting gel phase with a metal is described, which uses galvanic displacement. Electrolytes are dissolved in the gel phase to allow metal deposition from an immiscible liquid electrolyte solution. Conformal deposition was achieved by imprinting the gel, followed by galvanic displacement of gold.

Full text of DOI:10.1039/c1cc14472c

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COMMUNICATION
Metallisation of gel surfaces under ambient conditionsw
Huong L.T. Ho and Robert A.W. Dryfe*
Received 22nd July 2011, Accepted 4th September 2011
DOI: 10.1039/c1cc14472c
A room temperature method to coat a non-conducting gel phase
with a metal is described, which uses galvanic displacement.
Electrolytes are dissolved in the gel phase to allow metal
deposition from an immiscible liquid electrolyte solution.
Conformal deposition was achieved by imprinting the gel,
followed by galvanic displacement of gold.
reduction of Pd2+ to form nanoparticulate Pd nucleation
sites. Because of the miscible nature of the (aqueous-based)
gel and the aqueous deposition solution, diffusion of the
activating colloids through both phases is possible, hence
deposition is not necessarily restricted to a purely conformal
zone. An alternative approach to achieve the conformal
deposition of metal on an insulating substrate is reported
herein, where a gel phase containing a metal precursor is
placed in contact with an immiscible electrolyte solution.
Metal deposition can then be performed at this electronically
insulating surface. This novel method is based on the liquid–
liquid electrochemical approach, which can give interfacial
metal deposition either by electrolytic deposition7 or by spon-
taneous metallisation.8 Here this concept is extended to the
gel/immiscible electrolyte interface, the advantage being that
this ‘‘soft’’ interface can be stamped or moulded with patterns
whose image is retained in the conformal deposition process.
Agar is employed as the insulating mouldable substrate in the
present study. Conductivity is imparted to the gelled phase via
the salts dissolved to form a polymer electrolyte.9
The metallisation of the surface of insulators is a process of
considerable technological interest. Metallisation can be
achieved by methods such as chemical vapour deposition,
sputtering and evaporation, however one significant drawback
with such ‘‘line of sight’’ based methods is their inability to
achieve conformal deposition, i.e. to form uniform deposits
within recesses. Accordingly, there has been much interest in
the use of methods based on solution phase redox processes to
achieve metallisation, with control over the deposit structure
on the (sub-)micron scale a particular goal.1 Electrodeposition
has attracted much attention in recent years as a route to the
precise deposition of thin layers, a significant body of work
now exists on the control over this process to yield uniform,
conformal deposits.2,3 One intrinsic disadvantage of electro-
deposition, however, is the requirement that the deposition
substrate is a conductor.
The surface of the gel was covered with a Au deposit by a
spontaneous reduction of Au3+ by decamethylferrocene
(DmFc, dissolved in 1,2-dichloroethane, 1,2-DCE, see
supporting information for detailsw). Once deposited, the Au
film was used as a working electrode: a Pt wire (Advent,
diameter 0.5 mm, 99.99%) was brought into contact with
the film, glued to the side of the glass cell with epoxy resin
(Aralditet) and left to dry. Once dried, the contact between
the Pt wire and the Au film was improved by a small amount
of conductive Ag paint (SPI Supplies).
An alternative approach is based on electroless deposition,
which in some circumstances allows the catalytic deposition of
metals on insulating surfaces (e.g. polymers, oxides).4 For
example, Abbott et al. have reported the electroless deposition
of Au onto the surface of micron-sized SiO2 particles, with
further functionalisation with alkanethiols to permit their use
as substrates for the reversible adsorption of enzymes.5
A
The solution for electrochemical experiments consisted of
p-chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, 5 Â 10À3
M, Lancaster, 98+%) and tetrabutylammonium perchlorate
(TBAP) (0.1 M) as the organic electrolyte in 1,2-DCE. The
pseudo-reference electrode was made in-house by oxidation of
a Ag wire (diameter of 0.75 mm, Advent, 99.99%) immersed in
a saturated solution of LiCl. Platinum gauze (Alfa Aesar,
99.9%) was used as the counter electrode. A commercial Au
working electrode (IJ Cambria Scientific, working diameter of
2 mm) was also used to compare against the deposited Au film.
For conformal growth on the gel, a solution of 3 Â 10À3 M
NaAuCl4 with 0.1 M LiCl and 0.1 M LiClO4 was heated in a
water bath with the agar as described above. On cooling, the
gel mixture was poured into a dish. Before the gel had fully set,
a stamp with a recognisable feature was pressed against the gel
and left until the gel was solid. This stamp was then removed
disadvantage with the application of the electroless approach
to insulating surfaces is that a prior ‘‘surface activation’’ step is
needed to form nanoparticulate centres, from which metal film
formation occurs. Notably, Smoukov et al. have reported the
electroless deposition of copper on gel surfaces moulded by
soft lithographic methods.6 Although able to yield complex
metallised shapes, which can be removed in some cases as free-
standing films, this approach still relies on the prior activation
of the surface, specifically with Sn2+ sensitisation followed by
School of Chemistry, University of Manchester, Oxford Road,
Manchester, M13 9PL, UK. E-mail: robert.dryfe@manchester.ac.uk;
Fax: +44 161 275 4598; Tel: +44 161 306 4522
w Electronic supplementary information (ESI) available: Gel and
solution preparation, and details of reagents. See DOI: 10.1039/
c1cc14472c
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11318 Chem. Commun., 2011, 47, 11318–11320
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Fig. 2 Optical image of the Au film deposited on the surface of the
gelled aqueous phase, scale bar = 0.5 mm.
active area, D is the diffusion coefficient of the probe molecule,
c is the concentration of the probe and n is the scan rate. All
other symbols have their usual meanings. This gave an apparent
diffusion coefficient for p-chloranil in 1,2-DCE solution, which
was then used to determine the active area of the deposited Au
film. Cyclic voltammograms for p-chloranil using the Au film
as a WE are shown in Fig. 3. In contrast to the commercial
electrode, where an ideal 0.06 V peak separation was seen, the
voltammograms obtained for the deposited film show a slight
increase in peak separation with increasing scan rate. The
apparent diffusion coefficient was found to be 7.4 (Æ0.4) Â 10À6
cm2 sÀ1 for the standard Au working electrode. Compton et al.
found the diffusion coefficient for p-chloranil in acetonitrile
Fig. 1 Electron micrographs at successively higher magnification of
the Au film galvanically formed on an aqueous gel; (a) scale bar of
1.0 mm, (b) scale bar = 50 mm, some cracks can be observed, (c) scale
bar = 5 mm, and (d) scale bar = 10 mm where a section of the Au film
has come away from the gel.
to leave a distinct impression on the gel. A 1,2-DCE solution
of DmFc and TBAP (concentrations as above) was poured on
top of the gel in an approximate 1 : 1 ratio. The Au deposition
process is rapid, occurring on a time-scale of minutes. The gel/
organic solution system was therefore left for one hour to
allow complete formation of the Au film. Following deposition,
the gel structure was probed using scanning electron micro-
scopy (SEM). For this, the gel samples were thinly sliced and
placed onto a carbon tab attached to an Al SEM stub. The
metal covered gels were prepared so that the minimum amount
of gel needed to preserve the structural integrity of the metal
film was analysed.
to be 1.7 Â 10À5 cm2 sÀ1 12
,
which yields a D in 1,2-DCE of
7.2 Â 10À6 cm2 sÀ1 if solvent viscosity is corrected using the
Stokes–Einstein equation (eqn (2)):
D = kT/(6pZr)
(2)
where Z is the viscosity of the medium, r is the hydrodynamic
radius of the solute and all other symbols have their usual
meanings.13
Voltammograms were recorded for the system with the
deposited Au at different scan rates and the peak current
dependence plotted to find A via eqn (1). For the solution of
p-chloranil, A was found to be 1.24 cm2, compared to the
geometric area of the deposited Au determined by the diameter
of the glass cell, of 2.01 cm2. It should be noted, however, that
the uncompensated resistance visible in Fig. 3 will depress the
peak currents and therefore means that the A value is an
underestimate. On closer inspection of the SEM images at
higher magnifications (Fig. 1), it can be seen that the Au film is
not continuous. The thickness of the Au film deposited was
calculated to be 0.67 mm, assuming a 100% reduction
The deposited Au was analysed via SEM, the micrographs can
be seen in Fig. 1. In particular, Fig. 1c and d show a section of the
Au film coming away from the gel and folding back on itself.
This allowed an approximate measurement of the Au film
thickness, which was in the region of 600 nm. It was found that
leaving the gels exposed to the air caused collapse of the gel, due
to evaporation of water, and consequent shrinkage and fracture
of the deposited Au film. The resultant optical image of the film
can be seen in Fig. 2. The deposited Au film was used as a
working electrode to test its integrity, by adding the solution of
p-chloranil in DCE above the Au film.
The voltammetric response was compared to the response of
the same system at a standard Au working electrode. p-chloranil
is known to undergo two successive electron transfer
processes, to the radical anion and then to the di-anion,
respectively,10 however, only the former reduction was probed
here as the di-anion can undergo unwanted side reactions. The
apparent diffusion coefficient of p-chloranil was calculated for
the standard system, using voltammograms recorded at different
scan rates and the Randles-Sevcik equation (eqn (1)):11
Ip = 0.4463(nF/RT)1/2A1/2D1/2
n
1/2c
(1)
Fig. 3 Cyclic voltammograms, obtained for a solution of 5 mM
p-chloranil and 0.1 M TBAP on the deposited the Au film, recorded
at different scan rates. Arrows indicate the direction of the scan and
vertical line indicates the starting potential.
where Ip is the peak current, n is the number of electrons
transferred per molecule, F is Faraday’s constant, A is the
c
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Chem. Commun., 2011, 47, 11318–11320 11319

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of Au therefore has occurred when the Cu–Ni coin has come
into contact with the gel containing the gold precursor cation,
with oxidation of the coin and reduction of the metal salt
occurring at the surface of the gel network to form the gold
film seen in Fig. 4b. Fig. 4c show the gold galvanically
deposited in the features of the coin, the letters forming the
words ‘‘five’’ and ‘‘pence’’ can clearly be distinguished from
the gold deposit. Concurrently, it is noted that the surface of
the coin has been oxidised and is discoloured.
A novel method of metallising a soft insulating phase via
galvanic displacement is reported. The incorporation of
electrolytes into a polysaccharide gel imparts ionic conductance.
This allowed metallic deposition to be performed at the
surface of the gel. Au3+ species dissolved in the gel are
deposited as metallic gold on the gel surface upon contact
Fig. 4 Optical images of (a) the gelled aqueous phase containing
3 mM AuCl4À with 0.1 M of LiClO4 and 0.1 M LiCl that has an image
stamped into it. The black square outlines the imprinted image before
the deposition reaction and the (b) image recorded after metallisation
by reaction between the gel and 9 mM DmFc and 0.1 M TBAP in
DCE. The cell diameter is 0.564 cm. (c) SEM images of the Au
deposited into the features imprinted from a 5 pence coin.
with an immiscible electrolyte solution containing
reducing agent.
a
The deposited film could be used as an electrode; although
electrically conducting, the voltammetric analysis indicated
that the active area was smaller than the geometric area of
the gel surface. Traditional techniques of metal coating such as
sputtering, deposition and film formation from molten metals
require a vacuum chamber and/or high temperatures, and are
not readily applicable to conformal deposition. It has been
demonstrated that coating a gel with a pattern imprinted on its
surface is possible with galvanic displacement at room tem-
perature without the need for a vacuum chamber. Imprinting
ionically conductive gels with features in the size range of
hundreds of microns to centimetres, and successfully deposit-
ing gold within these features has been achieved without the
need for additives or catalysts. Applications of the deposited
film, for example as electrodes in gel electrophoresis, are
currently being explored.
efficiency, based on the amount of Au ion added to the gel,
while the final deposit thickness was measured to be 0.6 mm
(Fig. 1c).
While the maximum area is determined by the cell in use, it
should be noted that the preparation of the film involved exposing
the gel, whilst the epoxy resin and conductive paint dried,
meaning the gel would have shrunk in size due to water loss.
This would decrease the size of the active area, as the Au film will
also break up as the gel shrinks. Analysis of the film using SEM,
even under a low vacuum, would however have caused greater
shrinkage of the gel and breakage of the Au film. It is, therefore,
unclear from the SEM analysis whether the gaps and cracks seen
in the film result from gel shrinking, either naturally or under
vacuum for microscopic analysis, a combination of both, or
whether the Au film forms in this state on a gelled medium.
Conformal growth of Au was achieved on a gelled electro-
lytic phase; Fig. 4a shows an image recorded for the gelled
aqueous phase with an imprint of the stamp’s features prior to
the deposition reaction. The highlighted black square
illustrates where the rubber stamp was positioned before the
gel had fully set. Fig. 4b shows the galvanically displaced Au
coated image of the stamp. The organic phase containing the
reducing agent was specifically placed inside the indentation
left by the stamp, thus in Fig. 4b, the gold covered image can
be seen surrounded by the bare gel surface. The deposited Au
is formed in the channels left by the stamp and so the image
can be seen clearly. A coin (British decimal five pence coin)
was used as a stamp with features smaller than the patterned
stamp, shown in Fig. 4c. The coin, following cleaning, was
held in place on top of a cooling gelled aqueous phase
containing the gold salt and electrolyte. Once the gel had set,
the coin was removed and resultant observation showed that
metallic gold was deposited where the gel and coin had come
into contact with each other. The coin is minted from an alloy
consisting of 75% Cu and 25% Ni.14 Spontaneous deposition
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11320 Chem. Commun., 2011, 47, 11318–11320
This journal is The Royal Society of Chemistry 2011