Electrochemical Sensors / Biosensors Based on Carbon Aerogels / Xerogels Liana Maria Muresan * and Aglaia Raluca Deac “Babes -Bolyai” University… [612603]

1 Chapter Number
Electrochemical Sensors / Biosensors Based on
Carbon Aerogels / Xerogels

Liana Maria Muresan * and Aglaia Raluca Deac
“Babes -Bolyai” University
Department of Chemical Engineering
11, Arany Janos St.
400028 Cluj -Napoca
Romania
Tel:+ 40 264 595 872
E-mail of first author: [anonimizat]
E-mail of second author: [anonimizat]
*Corresponding Author

1. Introduction
Carbon aerogels and xero gels are microporous carbon materials that have received
considerable attention in the literature over the past decade [Aragay 2012, Wang 2005, Rodrigues
et al. 2011 .]. These materials have exceptional properties such as high porosity, tunable surface
area and pore volume, adjustable pore structure, controlled pore size distribution, low electrical
resistivity and very good thermal and mechanical properties [ Pekala et al. 1998 ]. They can be
produced in different forms (as powder, thin -film, cylinders, spher es, discs, or can be custom –
shaped) depending on their applications [Rodrigues et al. 2011 ].

2 The particularly interesting properties of aerogels / xerogels recommend them for a wide
range of applications such as electrode materials for capacitors or superc apacitors [ Lee et al.
1999 ], adsorption materials for gas separation [Meena et al 2005 ], catalyst supports [ Kim et al.
2008 ], column packing materials for chromatography [ Steinhart et al. 2007 ] biotechnolog ical
applications [Liang et al. 2003 ] and sensing devices [ Chu and Lo 2008 ].
The electrochemical sensing devices based on carbon aerogels / xerogels are excellent
sensing platforms with electrocatalytical activity and molecular recognition capabilities which
exploit the outstanding properties of these ma terials. In this context, recent advances in the
preparation and characterization of electrochemical sensors using carbon aerogels / xerogels are
reviewed with an emphasis on their application in heavy metals detection . The challenges for
future research a re also briefly discussed.

2. Aerogels and xerogels
Aerogels are mesoporous nanomaterials in which there is a high volume of free space
networked with nanometer solid domains [Rolison and Dunn 2001 ]. They are “empty materials”
which belong to a special class of synthetic porous ultralight materials [ Aegerter et al. 1990 ]. An
aerogel is obtained by replacing the liquid phase of a gel by a gas in such a way that its solid
network is retained, with only a slight or no shrinkage in the gel. It was firstly achieve d in the 20th
century by Kistler under supercritical conditions [Kistler 1931 ] but it is now possible under
ambient drying conditions as well. The s hrinkage in the case of aerogels is less than 15%.
A xerogel is an open network formed by the removal of al l swelling agents from a gel. It
is obtained when the liquid phase of the gel is removed by evaporation. It may retain its original
shape, but often cracks due to the extreme shrinkage (more than 90%) are experienced while
being dried. Therefore the method of drying will dictate whether an aerogel or xerogel will be
formed [IUPAC 1997 ].

3 When the liquid solvent is removed by freeze -drying, a cryogel is obtained [Lozinski et
al 2003 ]. Like aerogels, the cryogels have a meso – and macroporous structure dependin g on the
experimental factors controlling the first preparation step. The resulting structure after low
temperature pyrolysis of the gel is mainly meso – and microporous, while at higher temperatures,
the structure is mostly macroporous.
Aerogels differ fr om xerogels mainly by the surface area and their porosity. Thus,
aerogels have higher surface area and total pores volume than the xerogels prepared from the
same precursor [ Moreno -Castilla and Maldonado -Hodar 2005 ].
Only a limited number of materials can be obt ained under the form of aerogel / xerogel.
The most representatives are oxide -based aerogels (composed of silica -oxygen or metal -oxygen
bonds), organic aerogels (consisting of resin -based and cellulos e-based aerogel ), carbon aerogels
(carbonized plas tic, CNT and graphene), chalcogenide aerogels and aerogels derived from natural
materials, like gelatin, agar, egg albumin and rubber etc. [Du et al. 201 3]. Among these, carbon
aerogels deserve special attention due to their multiple applications .

3. Prepar ation of aerogel s /xerogels
The first method used to prepare aerogels was supercritical drying . This method
consist s in drying assisted by the use of supercritical fluids, usually CO 2, leading to preserv ation
of the high open porosity and superior textural properties of the wet gel in its dry form.
Supercritical drying has major advantages in comparison with classical drying methods, avoiding
the presence of vapor -liquid intermediates and surface tensions in the gel pores and thus
preventing pore collapse p henomenon [ García -González et al. 2012 ]. Nevertheless, supercritical
drying does not always preserve the wet gel structure [ Job et al. 2005 ]. The experimental
conditions (pH, ratio of precursors etc.) s hould be carefully chosen in order to avoid shrinkage
and residual surface tensions.

4 In the 1960s, Teichner and Nicolaon prepare d aerogels by using the sol-gel method , a
much faster way to obtain these materials [Nicolaon and Teichner 1968 ]. Their work opened the
possibility to prepare a wide range of aeroge ls including metal oxides, composite and hybrid
materials etc. Nowadays, 90% of aerogels are obtained using sol -gel process .
The sol -gel method was first used to prepare inorganic materials such as ceramics or
glasses but it was rapidly extended to other m aterials such as oxides, organic compounds etc. It
consists in progressive hydrolysis and condensation reactions of molecular precursors in a liquid
medium in the presence of a catalyst [Brinker and Scherer 1990] . Precursors used in sol -gel
processing are metallic salts, alkoxides, organic precursors (formaldehyde, resorcinol, melamine ,
polyacrilonitrile etc.) which can be solved in water or in different organic liquids. Gelation
implies the transformation of a sol to a gel and can be strictly controlled by the factors affecting
the process (precursor/catalyst molar ratios, solvent nature, working temperature etc.). Gels are
often aged in the mother liquor, then they are washed and dried to obtain xerogels (by simple
evaporation), aerogels (by supercritical drying) or cryogels (by freeze drier) [ Job et al. 2005,
Pajonk 1995 ].
The advantages of sol-gel method are numerous: the processing temperature is low (often
close to the ambient one), the method is waste -free and excludes the stage of washing [ Wang and
Bierwagen 2009 ]. One of the reasons for using sol -gel technique is due the hydrolysis and
polycondensation reactions which enable a special relation between inorganic and organic
compounds on molecular level. On the other hand, this method can ensure the ob taining of
reproducible materials thanks to the possibility to adjust the reaction paramete rs [Tomina et al.
2011 ].
The hydrolysis and condensation processes, which occur prior to the formation of sol are
taking place according to the following simplified reactions [Bach and Krause 2003 ]:
M(OR) n + H 2O  M(OH) n + ROH (1)
where M= Al, Si, Ti, V, Cr, Mo, W, etc

5 2M(OH)n  MOn+H2O (2)
In real systems, the equations are more complicated, especially because the hydrolysis is
a multi -step process.
Carbon a erogels can be prepared starting from organic aerogels (e.g. resorcinol –
formaldehyde, or melamine -formaldehyde aerogels) by pyrolysis at temperatures exceeding 500
0C [Tamon and Ishizaka 2000 ]. The carbonization temperature influences significantly the
structural characteristics of carbon aerogels, affecting their microporosity, the surface area, the
pores dimensions and the pores size distribution [ Pierre and Pajonk 2002]. At low temperatures,
the mesopores and micropores volume s increase on behalf of macr opores, whereas at high
temperatures these volumes decrease.
By changing the precursor/catalyst molar ratio, the solvent nature or the catalyst, the
microstructure of carbon aerogel s can also be tailored. Thus, high precursor/catalyst ratios lead to
collo idal aerogels consisting of spherical particles, while low ratios give rise to polymeric
aerogels [ 16]. The sol –gel synthesis is quite flexible and permits the preparation of gels, and
finally ca rbon gels, in different formats: monolith, powder, grain, pel lets or coatings [Morales –
Torres et al. 2011 ].
The most common method to prepare carbon aerogels is based on polymerization of
resorcinol and formaldehyde, by using sodium carbonate as basic catalyst and deionized water as
solvent [Halama and Szuzda 2010 ]. Recently, by replacing water with acetone and sodium
carbonate with perchloric acid, fractal carbon aerogels were prepared from resorcinol –
formaldehyde precursors [ Barbieri et al. 2001 ].
To prepare carbon xerogels, different drying methods can be used. F or example
microwave drying were used to obtain porous carbon xerogels. Using this method the procces to
obtain these materials is simplified, time is reduced, no pretreatment is needed and textural
properties are controlled. Although evaporation is the co mmon method reported by literature, the

6 using of this produce shrinkage in initial gel structure , while microwave drying method offers
certain advantages [ Zubizarreta et al . 2008 ].
Graphene based aerogel s [Ji et al. 2013 ] can be prepared under strictly co ntrolled
conditions and offer distinctive opportunities for creating highly conductive composite material s.
Their outstanding capacitive behavior is attributed mainly to the high degree of graphitization,
large specific surface area and pore volume, and co nvenient pore size and pore size distribution .
Cross linking and annealing are ways to significantly improve the mechanical properties and the
surface area of these materials [ Kohlmeyer et al. 2011 ]. An interesting way to prepare g raphene –
based carbon aero gels is through the carbonization of chitosan aerogel and activation with KOH.
The electrochemical behavior of graphe ne-based aerogels suggested that the ir 3D structure with
well-developed and interconnected pore network was reported to provide ionic chann els for
electrochemical energy storage [ Ji et al. 2013 ]. 3D graphene aerogel with honeycomb -like porous
structure and high C/O ratio was successfully prepared from graphene oxide dispersions in
isopropanol/water solution by simple γ -ray irradiation and fre eze-drying processes [ He et al.
2016 ]. Another simple method to prepare “pure” graphene aerogel is via reduction/self –
crosslinking of graphe ne oxide dispersion induced by L -ascrobic acid and drying of the wet
graphene gel [ Zhang et al 2011 ].
Another kind o f interesting carbon aerogel is carbon nanotube (CNT) aerogel. Carbon
nanotubes based aerogels [ Kohlmeyer et al. 2011 , Bryning et al. 2007 ] can be obtained from
aqueous gel by critical -point drying or lyophilization of precursors. They can be reinforced fo r
example with with PVA polymer [ Vigolo et al. 2000 ] which sensibly improves the strength and
stability of the aerogel. Another possibility is to prepare CNT -based aerogels starting from a gel
consisting of a 3D chemical assembly of CNTs in solution with a chemical cross -linker; followed
by CO 2 supercritical drying and thermal annealing [ Kohlmeyer et al. 2011 ]. Thermally annealed
CNT aerogels are mechanically stable, highly porous , exhibit excellent electrical conductivity and
large specific surface area.

7 Sol-gel process can be used to obtain template -oriented carbon aerogel by growing carbon
on a lattice (template) of a zinc oxide crystals, and then the zinc oxide is removed in an oven
leaving just the carbon aerogel [Gao et al. 2007 ].

4. Doped carbon aerogel s /xerogels
Aiming at modifying the structure, catalytic activity or conductivity of carbon aerogels ,
dopant element s are introduced into the carbon framework by different methods. Dopants can be
metals ( e.g. Fe [ Fort et al. 2013 , Gligor et al 2013 ], Bi [ Deac et al. 2015 a, Deac et al. 2015 b, Fort
et al. 2015 ], Pt [ Maldonado -Hodar et al. 2004 ], Ag [ Sanchez -Polo et al. 2007 ], Co [ Tian et al.
2010 ], Cr [ Moreno -Castilla and Maldonado -Hodar 2005 ] etc) but also non metals such as
Nitrogen [ Barbosa et al. 2012 ].
The dopant can be introduced in the aerogel framework either in the initial precursor –
solvent mixture, during the gelation step or after the carbon aerogel is formed ( e.g. by surface
deposition) [Moreno -Castilla and Maldonado -Hodar 2005 ]. The dopant can change the chemistry
of the sol -gel process by catalyz ing the polymerization or gelation process and consequently,
influence s the surface morphology and pore texture of the aerogels , as well as their particles size
[Moreno -Castilla and Maldonado -Hodar 2005 ]. The initial pH of the precurso r solution also
influences the chemistry of the aerogel -producing process. Thus, the experimental conditions
should be carefully controlled.
Metal doped carbon aerogels are better electrocatalysts than “pure” aerogels . Howeve r,
an important issue is the necessity of a homogeneous distribution of the metals inside the carbon
matrix in the form of nanoparticles. During the preparation of the gel should be avoided a
possible encapsulation of the metal particles in the matrix, whi ch leads to a restricted access of
gases and reactants to these particles, and consequently, to a diminished catalytic activity.
Nitrogen doped graphe ne aerogels exhibit improved electrochemical performance. For
example, g raphene -based nitrogen -doped poro us carbon aerogels obtained by carbonization of a

8 nitrogen -containing renewable biopolymer (Chitosan) [Hao et al. 2015 ] is a good electrode
candidate for construction of a solid symmetric supercapacitor, which displays a high specific
capacitance of about 197 Fg-1 at a current density of 0.2 A g-1. A new class of nitrogen -doped
carbonaceous nanofibers aerogels was prepared by hydrothermal carbonization with D (+) –
glucosamine hydrochloride as the precursor [ Song et al. 2016 ]. N-doped carbon xerogels can be
prepared also from a nitrogen -containing polymer precursor using melamine and urea as nitrogen
sources that were incorporated into the polymer matrix via the sol –gel process [ Barbosa et al.
2012 ].
The dopping of carbon aerogels with N is aiming to increase the basicity of the surface
and, consequently, the adsorption properties [ Botelho Barbosa et al. 2012 , Meng et al. 2014 ]. N-
doped carbon materials are efficient OER catalysts and promote H 2O2 electroreduction [ Cai et al.
2016 ].

5. Applications of carbon aero gels /xerogel
Carbon aerogels have a wide range of applications, varying from catalysis to environment
protection and electroanalitycal applications.
Thus, doped or undoped carbon aerogels can be used as catalysts or catalyst support in
water purification [Rasines et al. 2012 ] and for oxygen reduction reaction [Meng et al. 2014 1,
Nagy et al. 2016, Seredych et al. 2016 ]. Pt supported on carbon aerogels acted as catalysts in t he
toluene combustion reaction [Maldonado -Hodar et al. 2004 ] and in proton exchange membrane
fuel cells [ Smirnova et al. 2005 ]. Carbon aerogels doped with transition metals (Co(II), Mn(II)
and Ti(IV)) were used as catalysts in the photooxidation process of naphthalenesulphonic acids
[Sanchez -Polo et al. 2006 ] or in n -hexane conversion [ Maldonado -Hódar 2011 ]. In the later case,
the presence of a metal with different oxidation states improve d the catalytic efficiency of the
carbon aerogel through a synergetic effect , favoring alternately either cracking or the
aromatization reaction.

9 Carbon aerogels doped with transition metals are very active in NO reduction, in absence
of oxygen [Catalao et al. 2009 ] and those doped with rare -earth metals are good catalysts for
Michael addition reaction [ Kreek et al. 2014 ].
Carbon aerogels can be used as adsorbents for the removal of various chemical species.
Thus, the adsorption capacity of carbon aerogel electrodes has been tested in NaCl solutions of
several concentrations in order to evaluate the possibilities to use them for water desalination
[Rasine s et al. 2012 ]. They were used also in the removal of organic dyes [Lin et al. 2015 ], and of
other pollutants such as Bisphenol A from aqueous solutions [ Hou et al. 2015 ]. Three –
dimensional nitrogen -doped graphene aerogels based on melamine were reported a s excellent
adsorbents for several metal ions such as Pb2+, Cu2+ and Cd2+ and recycling performance for the
removal of various oils and organic solvents. [Xing et al. 2015 ].
Due to their 3D structure, high surface area, low electrical resistivity and excel lent
electrical conductivity, carbon aerogels can be used as electrode materials for batteries and
supercapacitors (electrochemical double layer capacitors). In these devices, the storage of
electricity is achieved trough the charging of electrical double layer existing at the
electrode/electrolyte interface which is dependent on the porosity and surface area of the
electrode material [ Moreno -Castilla et al. 2012 ]. Various metal (Co, Cu, Fe, and Mn) -doped
carbon aerogels combine pseudo -capacitive property o f metal oxide with electrochemical
properties of carbon aerogel. The performance of carbon aerogels as supercapacitors depend s
strongly but not exclusively on the molar ratio of precursor to catalyst [ Li et al. 2015 ]. The
relation between the electrochemic al capacitance and the pore structure was widely investigated
in the last decade, and the results reported in the literature show the proportionality between the
electrochemical capacitance and specific surface area for the same kind of carbon materials. More
than that, according to the literature data, the capacitance depends also on the pore volume, pore
size distribution, particle size, electrical conductivity as well as the electrolyte composition and
surface functional groups of the electrode materials [Halama et al. 2010 ].

10 One of the most widespread applications of carbon aerogels is in the field of
electrochemical sensing devices. In order to build such devices, new electrode materials based on
carbon aerogels were developed. The fabrication and appli cation of these electrode materials can
be achieved by several methods, shortly described in what follows.

6. Electrochemical sensors / biosensors based on C aerogels / xerogels
It is common knowledge that the electrode materials play a critical role in th e construction
of high -performance electrochemical sensing platforms for detecting target molecules through
various analytical principles. Aerogels/xerogels -based materials have great potential of improving
both selectivity and sensitivity of electrochemic al sensors and biosensors. Besides their attractive
characteristics described above they have the ability to act as effective immobilization matrices
and often generate a synergic effect among catalytic activity, conductivity, and biocompatibility
to accel erate the signal transduction and to amplify biorecognition. This is why they are used as
materials to prepare electrodes used in electrochemical detection of molecular and ionic species.

6.1 Preparation of carbon aerogel / xerogel modified electrodes
The carbon aerogel / xerogels can be used as modifiers of conventional electrode
materials (especially carbon based) either by surface or bulk phase modification.
Surface modified electrodes
Surface modification of a conducting electrode material is one of th e ways frequently
used to prepare carbon aerogel based modified electrodes. The substrate is generally a
conventional electrode material, such as glassy carbon, boron -doped diamond, graphite etc. and
the immobilization methods include the use of a polymer (Nafion, Chitosan, polypyrrole etc.),
molecular imprinting, sp raying etc.

11 The most used technique consists in the immobilization of the aerogel on the electrode
surface by using a polymer, either preformed, or obtained by the in situ polymerization of a
monomer. For example, a catalyst ink containing N -doped carbon nanofiber aerogel prepared by
blending the catalyst powder with Nafion and ethanol was deposited on the surface of a polished
glassy carbon electrode and tested for oxygen reduction [ Meng et al. 2014 ].
A novel electrochemical sensing platform with electrocatalytical activity and molecular
recognition capabilities was developed for the detection of dopamine, based on the modification
of molecularly imprinted polypyrrole onto a carbon aerogel surfa ce by electropolymerization and
molecular imprinting techniques [ Yang et al. 2015 ]. The film thickness on carbon aerogel can be
controlled by the electropolymerization time.
Composite aerogels incorporating layered MoS 2 nanoflowers and AuNPs were
immobiliz ed on the surface of glassy carbon by using chitosan in order to obtain a novel
electrochemical aptamer -based biosensor [Fang et al. 2015 ]. Chitosan was also used to
immobilize a Bi doped carbon xerogel on the surface of glassy carbon for Pb(II) and Cd(II) traces
voltammetric detection [ Fort et al. 2015 ].
Another technique is based on the deposition of the active material containing aerogels
on the surface of a conventional electrode material by spraying. Platinum/carbon aerogel
composites were immobilized on a boron -doped diamond (BDD) electrode by ink spreading
followed by drying. The modified electrode was used to prepare an acetyl cholinesterase (AChE)
based biosensor in order to detect organophosphorous pesticides [ Liu et al 2014 ].

Bulk modified electr odes
The most common bulk modified electrodes are carbon paste electrodes (CPEs) due to
their well -known advantages such as conductive entrapping matrix , low background current, wide
potential window, versatility etc. [ Gorton 1995 ]. A number of recent stud ies have been reported

12 on the electrochemical performances of CPEs incorporating carbon aerogels / xerogels [Gligor et
al. 2013 , Botelho Barbosa et al. 2012 ].
Carbon paste electrodes (CPEs) are popular because carbon pastes are easily obtainable at
minima l costs and are especially suitable for preparing an electrode material modified with
admixtures of other compounds thus giving the electrode certain pre -determined properties .
Carbon paste are prepared by thoroughly mixing of graphite powder with either p araffin / mineral)
oils or silicone fluids (as binder) until homogenization. The mixture is put into a cavity of a
Teflon holder and pyrolytic graphite in the bottom can be used for electric contact.
Carbon paste electrodes modified with carbon aerogel ca n be prepared by mixing
graphite powder with undoped or doped aerogel and paraffin oil. Thus, a carbon paste electrode
incorporating iron doped carbon aerogel was prepared and used for H 2O2 amperometric detection
[Gligor et al. 2013 ]. CPEs modified with Au NPs and decorated with Hemoglobin were used as
transducers for two novel biosensors exploiting the synergetic effect with ionic liquids, and their
catalytic ability to detect H 2O2 and NO 2−was studied [ Peng et al. 2015 ].
A new composite electrode consisting of carbon paste modified with carbon xerogel
containing Bi nanoparticles (BiCXe) was reported as efficient for the Pb2+ ions determination at
trace levels by using square wave anodic stripping voltammetry [ Deac et al. 2015a ]. It was
prepared by polyconden sation of resorcinol and formaldehyde in the presence of ammonium
hydroxide and glycerol formal followed by impregnation with a Bi salt, drying and pyrol ysis The
analytical parameters of BiCXe -CPE were satisfactory when compared with the maximum
admissible concentration of 10 μg Pb2+/l required by the UE legislation for the drinking water
[World Health Organization 2008 ]. The favorable performance of BiCXe -CPE is very attractive
also for environmental monitoring of toxic Cd(II) ions [ Deac et al. 2015b ] and for the
simultaneous detection of Pb(II) and Cd(II) ions.

13 Carbon paste modified with Fe doped mesoporous carbon aerogel (Fe -CA) was prepared
by using the sol -gel method coupled with the supercritical drying with liquid CO 2, and followed
by thermal pyrolysi s. Fe -CA was used for H 2O2 electrocatalytic reduction [ Fort et al. 2013 ].

6.2. Applications of e lectrochemical sensor/biosensors based on carbon aerogels/xerogels

Electro chemical sensor s are device s that transform chemical information, ranging from
the conc entration of a specific sample component to total composition analysis, into an electrical,
analytically useful signal. Biosensors are integrated receptor -transducer devices, which use a
biological recognition element to provide selective quantitative or s emi-quantitative analytical
information [ Thevenot et al. 2001 ].
As already mentioned, c arbon aerogels and xerogels are interesting materials for the
construction of electrochemical sensors and biosensors, as they posses excellent conductivity,
may promote the electrochemical oxidation of molecules on the electrode surface, and facilitate
the charge transfer in the redox reactions [ Wu et al. 2010 ]. On the other hand they have superior
characteristics such as low mass density, controllable porosity, large spe cific area and good
mechanical properties [ Moreno -Castilla et al. 2012 ] that can be very useful in the construction of
sensing devices.
Metal doped carbon aerogels combine the catalytic and conductive properties of metals
with the large surface area and p orosity of aerogels, offering an attractive sensing platform for
detection of various analytes . Among these, Bi-modified carbon xerogels offer one example of
successful electroana lytic application of metal -doped carbon aerogels in the field of heavy metal
ions determination. As Bi is considered one of the less toxic metals , Bi-bulk and Bi -film
electrodes are used as alternative to replace toxic mercury electrode in detection of heavy metals
ions from aqueous solutions. The use of Bi electrodes in is based o n Bi ability to form low

14 temperature alloys with heavy metals, [Kirk-Othmer 1978 ] favoring the accumulation of these
ions during the preconcentration step of the stripping analysis.
The preparation of Bi -modified carbon xerogels (BiCXe) by sol -gel method involves
several steps [Deac et al. 2015a ] (Figure 1):
(i) polycondensation of precursors (resorcinol and formaldehyde in the presence of
ammonium hydroxide and glycerol formal)
(ii) impregnation of the resulting resorcinol -formaldehyde gel (RF -gels) with Bi3+ salt
(iii) gel drying by evaporation
(iv) pyrolysis of the Bi impregnated organic xerogel in Ar atmosphere

Figure 1. Steps involved in Bi -doped carbon xerogel preparation by sol -gel method

One way to prepare electrode materials based on Bi doped carbon xerogels is by
incorporating them into carbon paste. The BiCXe -carbon paste electrodes exploit successfully the
favourable electrochemical properties of carbon paste electrodes (large potential window,
versatility etc.) and the unique electroanalytical characteristics of Bi-based xerogels (e.g. Bi-
based electrodes are less susceptible to oxygen interference, exhibiting a lower background
current for square wave anodic stripping voltammograms [ Lee et al. 2007 ]). Carbon paste
electrodes are prepared using graphite powder and paraffin oil in which different other materials
of interest can be introduced. Thus, BiCXe -CPEs were used for the successful determination of
Pb2+ and Cd2+ ions at trace levels by using square wave anodic stripping voltammetry (SWASV)
[Deac et al. 2015a ]. In anodic stripping voltammetry, the analyte of interest is electrodeposited on
the working electrode during a reduction step, and oxidized from the electrode during the
stripping step, the resulting current being measured.

15 Figure 2. SWASVs responses for increasing concentrations of Cd2+ and Pb2+, and the
corresponding calibration curves at BiCXe -CPE electrode, (Cd2+ circle, square for Pb2+,
respectively) (inset). Experimental conditions: electrolyte, 0.1 M acetate buffer (pH 4);
starting potential, -1.3 V vs. Ag/AgCl, KClsat; deposition time 120 s under continuous
stirring; scan rate, 0.050 V/s; frequency, 25 Hz; amplitude, 0.05 V; step potential, 0.004
V; equilibration time, 10 s

Anodic s tripping analysis with BiCXe -modified electrodes lead to wide line ar dynamic
range and a low detection limit s (0.18 g L-1 Pb2+; 0.045 μM L-1 Cd2+). Other advantages are easy
handling and modification, low toxicity and low cost, which recommend them for the trace
analysis of Pb2+ and Cd2+.
Another interesting development of the electrode materials based on carbon
aerogels/xerogels i s by combining them with different polymers. The high surface area of carbon
aerogels can provide much more multidimensional spaces for electrochemical modification of
conducting polymers, such as polypyrrole [ Fang et al. 2015 ], polystyrene [ Zhang et al. 2 008],
Nafion [ Brigaudet et al. 2008 ] Chitosan etc. [Zuo et al. 2015 ].
In the case of biosensors a challenge consists in loading the biomolecules and keeping in
the same time their bioactivity, which is often a difficult task. The large effective surface ar ea and
the porosity of aerogels allow also the immobilization of biomolecules at or near the electrode
surface, which facilitates the charge transfer between the electrode and the substrate [ Fang et al.
2015 ]. The effective reaction area is increased, whil e the electron transfer and ion diffusion paths
are hindered , which will lead to better electrochemical performance. Thus, some efficient
biosensors could be obtained, aiming to detect hemoglobin [ Peng et al. 2015 ], H 2O2 [Dong et al.
2013 ], organophosphoro us pesticides [ Liu et al. 2014 ] and others. In the construction of carbon –
based hybrid materials, metal nanoparticles such as Ni , Pd and Au are often preferred for their

16 high electrocatalytic behavior to biomolecules in sensing application [ Chen et al. 201 2, Naruse et
al. 2011 ].
Carbon aerogels can be also used for the obtaining of composite and hybrid electrode
materials with good electroanalytical performances. Hence, CA incorporated layered MoS 2
nanoflowers were prepared by using a facile hydrothermal ro ute and were used to prepare a novel
electrochemical aptamer -based biosensor [Fang et al. 2015 ]. Molecularly imprinted TiO 2/carbon
aerogel was used as electrode material for the photoelectrochemical determination of atrazine
[Zhang et al 2015 ], while condu ctive aerogels composed of carbon nanotubes (CNTs) and
cellulose were used as vapor sensors [ Qi et al. 2015 ].
The development of reliable sensors for real -time tracing of hazardous volatile organic
compounds and for monitoring of some specific pollutant ga ses in industries is of big importance .
Among these, s ensor films are widely used. Carbon aerogel thin film composites with different
sensitivity toward toluene, n -hexane, acetone, and water vapors at room temperature were
prepared and characterized [ Thubs uang et al. 2015 ]. Activated carbon aerogel/polymer
composites with appropriate polymer matrices exhibited excellent sensitivity and fast response
towards organic vapors .
Other examples illustrating different applications of carbon aerogels/xerogels -based
electrochemical sensors/biosensors are presented in Table 1.

Table 1. Carbon aerogels/xerogels -based electrode materials for electrochemical detection
of different chemical species

No Electrode material Electrolyte Analyte Method Ref.
1 Fe doped C aero gel – CPE 0.1 M phosphate buffer
solution (pH 7.0). H2O2 CV [Fort et al. 2013,
Gligor et al. 2013 ]

17 2 N-doped C xerogel 0.1M KCl solution K3Fe(CN)
6 CV [Barbosa et al.
2012 ]
3 N-doped C nanofiber aerogel 0.1 M KOH solution O2 CV; RDE [Meng et al. 2014 ]
4 3D nitrogen -doped graphene
aerogel 0.2 M PBS, pH 7 H2O2 cyclic
voltammetry [Cai et al. 2016 ]
5 Mesoporous carbon aerogels
with 0 -14 % nitrogen
content
H2SO 4 5M oxygen
reduction
reactions Cyclic
voltammetry [Nagy et al. 2016 ]
6 C aerogel phosphate buf fer
solution Bisphenol
A electro –
polymerization [Hou et al. 2015 ]
7 C aerogel with molecularly
imprinted pPy 0.1 M phosphate buffer
solution (pH 7.0). dopamine DPV [Li et al. 2008 ]
8 Dual aptamer biosensor
using AuNPs/MoS 2/C
aerogel
phosphate buffer
solution;
human serum samples platelet –
derived
growth
factor BB
(PBGF –
BB) CV ; DPV [Fang et al. 2015 ]
9 Biosensor based on
platinum -carbon aerogel
composite PBS pH 7.3 organoph
os-phorus
pesticides differential
pulse
voltammetry [Liu et al. 2014 ]
10 AuNPs -C aerogel –
Hemoglobin 0.1 M phosphate buffer
solutions H2O2;
NO 2- EIS; CV;
amperometry [Peng et al. 2015 ]
11 Molecularly imprinted TiO 2 0.1M KCl solution atrazine photo – [Zhang et al. 2008 ]

18 / C aerogel electrochemica
l analysis
12 Inorganic/organic doped
carbon aerogel (CA)
materials (Ni -CA, Pd -CA,
and Ppy -CA) 0.1 M phosphate buffer
solution (PBS, pH 7.0)
H2O2 EIS,
amperometry [Dong et al. 2015 ]
13 Carbon nanotube –cellulose
composite aerogels – vapour
sensing monitoring the
electrical
resistance
change [Qi et al. 2015 ]
14. Polybenzoxazine -derived C
aerogel composite – vapor
sensor:
toluene;
n-hexane,
acetone electrical
response
measurements [Thubsuang et al.
2015 ]
15 Glassy carbon electrode
modified with chitosan /
CNT aerogel -Cu2O@CuO NaOH solution Glucos e cyclic
voltammetry;
amperometry [Liu et al. 2015 ]
16 Nitrogen doped porous
carbon aerogel O2 and N 2 saturated 0.1
M KOH Oxygen
reduction Cyclic
voltammetry [Alatalo et al.
2016 ]
17 Carbon aerogels A simulated waste
water containing
phenol Removal
of phenol Electrochemic
al oxidation –
cyclic
voltammetry [Guifen et al.
2009 ]

19 * CPE = carbon paste electrode; NPs = nanoparticles; CA = carbon aerogel; CNT = carbon
nanotubes

7. Conclusion
Carbon aerogels belong to a very promising class of materials with uniqu e properties that
have a wide range of applications varying from environmental clean -up, to energy storage devices,
chemical sensors, catalysts, and many others.
Doping of carbon aerogels /xerogels either with metals or non -metals is considered
desirable, as it enhances the electrical conductivity , improved thermal/oxidation stability ,
advantageously modifies the surface morphology and pore texture of the aerogels, as well as their
particles size and improves the catalytic activity of the aerogels.
The elec trochemical s ensors and biosensors based on carbon aerogels represent viable
solutions to detect various analytes of practical interest .
The challenges for future research concerning carbon aerogels/xerogels include:
– elaboration of novel methods that can b e applied for the characterization of aerogels as well
as their gel precursors
– development of existing and potential applications in wide range of technological areas
– tailoring of aerogels/xerogels properties in order to satisfy the requirements of differ ent
beneficiaries
– cost-effective mass production of aerogels/xerogels
Concluding, development of carbon aerogels/xerogel -based materials is foreseen as a
promising and generous research field in the years to come.

Keywords: carbon aerogel s/ xerogesl; dop ed aerogels; electrochemical sensors/ biosensors; Bi-
doped carbon xerogel ; heavy metals detection.

20
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