Introduction
Mercury (Hg) is released into the atmosphere through human
activities, predominantly fossil fuel combustion, and naturally, for example,
from soil outgassing, volcanoes, and evasion from the sea (Pirrone et al.,
2010; Pacyna et al., 2010). One of the more troublesome issues in recent
years has been to quantify not only the strength of emission sources but also
the effects of the re-emission of previously deposited Hg on the overall
distribution, concentration, and speciation of Hg in the atmosphere (Hedgecock
et al., 2003). The deposition of atmospheric Hg depends on its chemical
speciation, where the term speciation is used to distinguish between the
gaseous elemental (GEM) and gaseous oxidized forms of Hg (gaseous oxidized mercury, GOM;
particle-bound mercury, PBM) and their chemical and physical characteristics (Lyman et al.,
2010; Sprovieri et al., 2016, 2017). To be precise, total gaseous mercury (TGM)
is mainly comprised of GEM with minor fractions of other volatile species (e.g.,
HgO, HgCl2, HgBr2, CH3HgCl, or (CH3)2Hg). However, in
spite of the conceptual differences between TGM and GEM, they have often been
used without clear distinction. This was allowable to a degree as the
predominant fraction of TGM (usually in excess of 99 %) is often
represented by GEM under normal conditions. GEM is relatively inert under
atmospheric conditions, only slightly soluble, and quite volatile,
whereas several oxidized Hg forms found in the atmosphere are both soluble
and involatile; thus they are efficiently scavenged and consequently
deposited by liquid atmospheric water, such as rain and fog droplets, but
also deliquesced aerosol particles. The dispersion of GEM on a global scale
therefore depends on the rate of its oxidation in the atmosphere as this
determines its long atmospheric lifetime (generally > 1 year),
limiting local emission controls in protecting all environments. Several
international initiatives and programs (i.e., the United Nations Environment
Programme; UNEP) have also made a tremendous effort to identify and
quantify Hg pollution across the globe, especially the “hot-spots”,
to reduce the risk of exposure to this neurotoxic pollutant. Policy
makers are working toward a worldwide effort to support the construction of
an accurate global Hg budget and to model the benefits or consequences of
changes in Hg emissions, for example, as proscribed by the Minamata
Convention. Anticipating a global policy, in 2010 the European Commission
began a 5-year project called the Global Mercury Observation System (GMOS;
www.gmos.eu) to create a coordinated global network to fill the gaps in emission
monitoring and in the spatial coverage of environmental observations, mostly
in the tropical regions and the Southern Hemisphere, thus adequately improving
models and making policy recommendations (Sprovieri et al., 2016, 2017). To
date,
the GMOS network consists of more than 43 monitoring stations worldwide,
including high-altitude and sea-level monitoring sites
located in climatically diverse regions, including the polar areas (Sprovieri et
al., 2016, 2017). One of the major outcomes of GMOS has been an interoperable
e-infrastructure developed following the Group on Earth Observations (GEO)
data sharing and interoperability principles, which allows us to provide
support to UNEP for the implementation of the Minamata Convention (i.e.,
Article 22). GMOS activities are currently part of the GEO strategic plan
(2016–2025) within the flagship project to track persistent pollutants. The
overall goal of this flagship project is to support the development of GEOSS (Global
Earth Observation System of Systems) by fostering research and technological
development on new advanced sensors for in situ and satellite platforms in
order to lower the management costs of long-term monitoring programs and
improve the spatial coverage of observations. Since automated measurement methods
for Hg often require power, carrier gases like argon, and significant operator
training, they are difficult to apply for understanding Hg air concentrations
and deposition across broad regional and global scales. Therefore, the lack
of an inexpensive, stand-alone, low-power, and low-maintenance sensor is a
primary technical issue to be solved for the sustainability of a global
network such as GMOS. Previous research has highlighted the fact that Hg concentration
levels in air vary greatly across different environmental locations; these include remote
polar regions, background or rural, and urban locations with an
average range between 1.5 ngm-3 (GEM) and 1 pgm-3 (GOM and PBM) depending on the speciation.
Hence, for the determination of atmospheric Hg at such low levels,
sampling and analytical methods should be sensitive enough to quantify the
concentration profiles of diverse Hg species in each respective environmental
setting to better understand their environmental behavior and patterns.
Fortunately, many advances in analytical methodologies have made it
possible to study atmospheric Hg in different environmental locations.
However, several limitations and difficulties are still experienced in Hg
analysis, as most methods cannot yet directly or accurately determine minor
Hg species (Gustin et al., 2013). Hence, efforts should be continued to
further secure the reliability, traceability, and accuracy of Hg
levels measured in air. Current air-monitoring devices are amply sensitive to
detect the global background but are costly and high maintenance with complicated
configurations and
electricity requirements. A further limitation is the
ultralow levels of ambient mercury in the atmosphere. The typical background
gaseous elemental mercury (GEM) level of 1.5 ng m-3 is equivalent to 168 parts
per quadrillion by volume (ppqv). There is no other atmospheric compound
being measured routinely, continuously, and automatically at this ultralow
concentration. These features limit the scientific research community's
long-term ability to measure atmospheric Hg concentrations worldwide.
Sampling and analysis of atmospheric Hg is conducted most commonly as GEM and/or TGM
because of their greater abundance, even though both manual and automatic methods
have been developed for different Hg forms to suit the measurement
and monitoring application. The most common sampling method employed relies
on adsorption on gold amalgam and then, either directly or indirectly,
through a stepwise process of thermal desorption and final detection (usually
by cold-fiber atomic absorption spectroscopy, CVAAS, or cold-fiber atomic
fluorescence spectroscopy, CVAFS). However, there are several gaps in our current knowledge to be solved. Firstly, the atmospheric chemistry of Hg remains
poorly understood, especially the oxidation pathways by which GEM is
converted to GOM, the reduction pathway that converts GOM back to GEM, and
the gas–particle partitioning. This is partially due to the need for
the identification of the chemical forms of oxidized Hg in the atmosphere and
the methods to measure these compounds individually. In addition, the limitations
and potential interferences with our current measurement methods have not
been adequately investigated; thus alternate methods to measure atmospheric
Hg are needed. Given the uncertainty and restrictions associated with
automated and/or semi-automated Hg measurements (Gustin et al., 2013; Pirrone
et al., 2013), particularly in responding to the technical needs of an
expanding Hg global observation network, we developed a reliable, sensitive,
and inexpensive surface for atmospheric Hg detection. In particular, we
investigated and demonstrated the utility of composite nanofibrous
electrospun layers of titania decorated with gold nanoparticles (AuNPs) to
obtain nanostructured materials capable of adsorbing GEM as a useful
alternative system for making regional and global estimates of air Hg
concentrations. Previously developed methods and new sampling systems, such
as passive samplers, have been used to understand the long-term global
distribution of persistent organic pollutants (POPs; Harner et al., 2003;
Pozo et al., 2004). Other passive samplers for both TGM and GOM collection on
the basis of diffusion have been constructed using a variety of synthetic
materials (i.e., gold and silver surfaces, and sulfate-impregnated carbon)
and housings (Lyman et al., 2010; Gustin et al., 2011; Zhang et al., 2012;
Huang et al., 2014). However, because of the differences in the design of passive
samplers, ambient air Hg concentrations quantified by various samplers may
not be comparable. In addition, the sampling rates (SRs) using the same passive
samplers may depend on the environmental conditions and atmospheric chemistry at
each site. It has also been highlighted that the performance of
passive samplers may be influenced by meteorological factors (e.g., T
∘C, RH, and wind speed), therefore inducing bias to the results of
passive sampling (Plaisance et al., 2004; Sderstrm and Bergqvist, 2004). On the
other hand, an incentive for developing simple and cost-effective samplers that
are capable of monitoring over an extended period and require no technical
expertise even at remote locations is now
clear. In this work, we describe an alternative approach adopted in the
place of conventional ones, demonstrating that the combination of the affinity of gold
for Hg and a nanoscale-sized framework of titania provided the
chance to create promising sensors for environmental monitoring in real time
characterized by high sensitivity to the analyte. The novel sensor is a
relatively simple and low-cost method for the measurement of the most abundant Hg
form in ambient air (TGM or GEM) due to reusable parts and simple deployment
steps. Further, we have evaluated the applicability of this measurement
technique with respect to real environmental conditions, highlighting future
directions for research on airborne Hg determination. The TGM–GEM sensor
surface described here could be deployed in a global network such as GMOS; a
permanent network of ground-based monitoring sites and observations of Hg
and/or related species on a global scale with remote sensors would in
fact be highly desirable. These data are needed to test and validate model
processes and predictions, understand the source–receptor relationships,
understand long-term changes in the global Hg cycle, and would help
policy makers to set regulations for different areas. The sensor features are
related to the nanofibrous scaffold of titania capable of growing gold
nanoaggregates through photocatalysis that are tunable in size and shape. Such a
nanostructured layer, fabricated with electrospinning technology, firstly
improves sensor features with respect to those of compact films, by enhancing
the global number of analyte–sensor binding sites and reducing some bulk
drawbacks. Secondly, the combination of metal oxides and metal
nanostructures improves the sensitivity, allows the sensor to work at room
temperature, tunes the selectivity towards different gas species by adjusting the
surface-to-volume ratio of nanosized structures, and affects the sensor lifetime.
Morphological, optical, and electrical aspects, as well as the sensing measurements of GEM fibers
in air have been reported and discussed. When designed, the resulting
Hg adsorbent or absorbent material was expected to be suitable for novel Hg sensor
fabrication, since a similar nanofibrous scaffold doped with AuNPs was
described in the literature as a filtering system capable of adsorbing and
removing
Hg vapor from the environment with an efficiency of nearly 100 % (Yuan et
al., 2012). In previous work (Macagnano et al., 2017,
2015a), the authors reported a high sensitivity of the sensor
capable of detecting up to dozens of pptv despite the long time needed to
reveal the analyte at these concentrations in air. In this work, the chance
to apply the sensor in polluted sites and in real time has been presented and
described.
Materials and methods
Chemicals
All chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further
purification: polyvinylpyrrolidone (PVP; Mn 1 300 000), titanium
isopropoxide (TiiP; 99.999 %), gold (III) chloride hydrate (HAuCl4;
99.999 %), anhydrous ethanol (EtOHa), and glacial acetic acid
(AcAcg). Ultrapure water (5.5 × 10-8 Scm-1) was
produced by Milli-Q EMD (Millipore Corporation, Darmstadt, Germany).
Electrospinning technology
Electrospinning (ES) is a widely used technique for the electrostatic
production of nanofibers, during which an electric field is used to make
polymer fibers with diameters ranging from 2 nm to several micrometers from
polymer solutions (or melts). It is currently the most economic, versatile,
and efficient technology to fabricate highly porous membranes made of
nanofibers
and/or microfibers for sensors (Macagnano et al., 2015b). It is based
on the application of a high-voltage difference between a spinneret ejecting
a polymeric solution and a grounded collector. The jet of solution is
accelerated and stretched by the external electric field while traveling
towards the collector, leading to the creation of continuous solid fibers as
the solvent evaporates. The electrospinning apparatus used in the present
study (designed and assembled in CNR laboratories) is comprised of a homemade
clean box equipped with temperature and humidity sensors, a syringe pump
(KDS 200; KD Scientific, Holliston, MA, USA), a grounded rotating cylindrical collector (45 mm diameter), and a high-voltage oscillator (100 V) driving a high-voltage
(ranging from 1 to 50 kV) high-power AC–DC (alternating current to direct
current) converter. The electrospinning solution (7.877 × 10-5 M)
was prepared by dissolving PVP in EtOHa and stirring (2 h). A 2 mL
aliquot of a 1:4 (w/v) solution of TiiP solved in a 1:1 (v/v) mixture of
AcAcg and EtOHa was freshly prepared and added to the 2.5 mL PVP
solution under stirring in order to obtain a 1.95 (w/w) TiiP / PVP final
ratio. Both mixtures were prepared in a glove box under a low humidity rate
(< 7 % RH). The syringe filled with the TiiP / PVP solution and
housed in the syringe pump was was connected to a positive DC voltage (6 kV)
and set perpendicular to a grounded rotating collector at 15 cm of distance. The
substrates were fixed through suitable holders onto the collector (600 rpm,
21 ∘C, and 35 % RH) and processed (feed rate 150 mL h-1)
for 20 min to obtain scaffolds for the sensors. After deposition, PVP / TiO2
composite nanofibers were left for some hours at room temperature to
fully undergo the self-hydrolysis of TiiP (Li et al., 2004) and then annealed under
an oxygen atmosphere (muffle furnace) using a thermal ramp from room
temperature up to 550 ∘C (1 ∘C min-1; 4 h dwell
time) in order to remove PVP and crystallize the metal oxide (anatase).
Transducers: interdigitated electrodes
The transducer adopted in the present work to convert the physiochemical
interactions of analytes with the different polymer fibers in an electrical
signal was an interdigitated electrode (IDE; Bakir et al., 1973; James et
al., 2013). Specifically, the transducer consisted of 40 pairs of
electrodes (150 nm in electrode thickness, 20 µm in gap and electrode
width, and 5620 µm in length); it was manufactured in CNR laboratories
through a standard photolithographic process (liftoff procedure)
followed by Ti sputtering and Pt evaporation suitable to generate
electrodes of the size reported above on a 4 in. oxidized silicon wafer.
After electrospinning deposition, all the electrical signals of the resulting
chemoresistors were recorded by an electrometer (Keithley Instruments, Inc., Cleveland, OH, USA; model 6517).
Titania nanofibers
Upon calcination, the diameters of the fibers extraordinarily shrank: the mean
diameters of the fibers were estimated through image analyses to be
approximately within the range of 60–80 nm. Specifically, the resulting
fibers appeared fine and rough at the surface with a fairly homogeneous fabric.
The absence of beads and the good quality of the long and continuous fibers
was confirmed through SEM micrographs. A highly porous and dense network of
nanofibers covering the electrodes was observed, showing interconnected void
volumes (porosity) and high surface-to-volume ratios (specific surface
area). Zampetti et al. (2013) reported that such a fibrous layer
showed 99 % of pores with an area of less than 10 µm2 with
more than 80 % pores being < 1 mm2.
AuNPs / TiO2NFs photocatalytic decoration
Exploiting the photocatalytic properties of TiO2, gold nanoparticles
were selectively grown under UV light irradiation on the electrospun
titania nanofibers through the photoreduction of HAuCl4 in the presence
of an organic capping reagent (PVP). The resulting fibrous scaffolds
were dipped into an aqueous solution containing HAuCl4 and PVP
(1.5 × 10-3 M and 0.1 M, respectively) and exposed to UV light
irradiation for specified intervals (UV lamp 365 nm; Helios;
Italquartz, Calenzano, Italy). Depending on the gold nanoparticle sizes that were
formed in photocatalysis, the dip solution changed from light yellow to
purple. Samples were rinsed extensively with water and then air dried.
Before the morphological, electrical, and sensing measurements, the samples were
heated at 450 ∘C per 1 h to eliminate the PVP traces.
Morphological characterization was provided through scanning electron microscopy
(SEM; Jeol, Tokyo, Japan; JSM 5200; 20 keV) with pictures captured at 5 kV of accelerating
voltage. AFM (atomic force microscopy) micrographs were taken in tapping
mode using 190Al-G tips, 190 kHz, and 48 N m-1 (Nanosurf AG, Liestal, Switzerland; FlexAFM). TEM
(C-TEM;
control transmission electron microscopy) micrographs were performed at 200 keV with an analytical double-tilt probe. TEM specimens were prepared by
gently scraping the TiO2 nanofibrous layer electrospun onto
the silicon support and then collecting the nanofibers through adhesion
upon contact with holey carbon thin film. UV–vis spectra were provided by
a spectrophotometer (UV-2600; Shimadzu Corporation, Kyoto, Japan) by analyzing quartz slices coated with
nanofibers. These substrates were able to collect fibers by electrospinning
(20 min) and were then subjected to calcination according to the procedure described
above; the final step was UV irradiation in the aqueous solution. The
fibrous layer stayed stuck to the substrate if the thickness was thin
enough. Longer depositions caused curling of the fibers during the calcination
process.
Measurement setup
The sensor was placed in a suitable PTFE-made measurement chamber (0.7 mL
volume) connected to an electrometer (Keithley 6517) capable of
measuring the current flowing through the IDE when a fixed potential
was applied to it and sending data to a PC. Dynamic measurements were
carried out at room temperature using (i) a four-channel MKS 247 managing
four MKS mass flow controllers (MFC) set in the range of 0–200 sccm and (ii) an Environics S4000 (Environics, Inc., Tolland, CT, USA) flow controller comprised of three MFCs
supplying different flow rates (up to 500, 250, and 25 sccm)
managed by its own software. Pure air (5.0; Praxair–Rivoira, Milan, Italy) was
used as a gas carrier. A homemade PTFE (polytetrafluoroethylene) permeation
tube filled with a suitable amount of Hg0 was included within the
delivery system to obtain set dilutions of Hg-saturated vapors. The tube was
immersed in a thermostatically controlled bath; thus the desired Hg0
concentration delivered to the sensor was achieved by tuning both the
temperature of the permeation tube and the dilution flow. The Hg0
concentration was checked by a Tekran® 2537A analyzer (Toronto, ON, Canada). Responses
were calculated as ΔI / I0, where ΔI was the current
variation and I0 was the current when synthetic pure dry air
flowed. The sensor was restored after a quick thermal shot at 450 ∘C
under a flow of pure air.
A sketch of an electrospinning setup comprised of a syringe
and a grounded rotating cylinder collector where the samples are placed for
coverage (a); a piece of TiiP / PVP nanofibrous
fabric removed from the substrate after 1 h of electrospinning
deposition (b); a reddish purple aqueous
solution of HAuCl4 / PVP after UV light irradiation
treatment and (c) a piece of the TiO2 (anatase) nanofibrous fabric
obtained after TiiP / PVP annealing.
A sketch of the photocatalytic process occurring on the
fiber surface (a); an SEM picture of a dense
nanofibrous network of AuNPs / TiO2 coating a silicon
wafer (b); and a C-TEM micrograph of fibers
finely decorated with gold nanoparticles (the darkest ones) bound without using any additional linker
(inset).
Results and discussion
Nonwoven mats made of PVP and amorphous TiO2 were obtained through the
combination of electrospinning and sol-gel techniques (Fig. 1). The ES
deposition proceeded for 20 min on oxidized silicon wafers and IDEs
properly fixed on the surface of a conducting and rotating collector to form
nanofibrous layers characterized by high surface areas and relatively small
pore sizes (Zampetti et al., 2013). By changing the deposition time, both the
thickness and the consistency of the mats changed. More specifically, a
1 h deposition provided the formation of a thicker, white, soft fabric that
is
hygroscopic, soluble in both water and polar solvents, and easily peeled off
(Fig. 1); instead, a 20 min deposition generated a fibrous film adhering to
substrates too thin to be weeded and thus preferred for sensor fabrication.
The calcination process caused a complete degradation of PVP with the formation
of crystalline TiO2 (anatase) and a significant shrinkage in the fiber
dimensions
(60–80 nm diameter; 5–40 nm grain size). By exploiting the photocatalytic
properties of titania (anatase), a tunable decoration of fibers with gold
nanoparticles could be achieved by dipping the fibrous mats in a proper
aqueous solution (HAuCl4; PVP) under UV light irradiation (Li et al.,
2004; Macagnano et al., 2015a). The photocatalytic reaction was confirmed by the
color change of the solution (reddish purple from light yellow; Fig. 1).
By changing both the UV irradiation exposure time and the PVP concentration as a capping
reagent, the morphology, size, and density of the gold nanoparticles could be tuned
(Macagnano et al., 2017).
In the present work, among a series of differently coated fibrous layers,
only the fibrous nanocomposites that were conductive at room temperature
were selected; their electrical and sensing features were then investigated.
The controlled gold deposition was due to the photo-excited electrons on the
surface of TiO2 nanofibers that were able to reduce the gold ions, thus
inducing gold metal deposition (Fig. 2 sketch). The capping reagent was
responsible for the shape of the particles. The surfaces of the nanofibers, as
observed in SEM micrographs (Fig. 2a), appeared densely decorated with
globular nanoparticles. In the C-TEM image (Fig. 2, inset) the gold nanoparticles
appeared darker with spherical or quasi-spherical shapes. The single
particle sizes ranged between 2 and 20 nm with a 7.8 ± 3 nm
average diameter (Macagnano et al., 2017). Gold nanoparticles grew directly
onto the nanofibers, and their adhesion appeared relatively strong (despite
van der Waals forces), since they resisted both water rinsing and
fiber scratching for TEM analyses.
As a result of the photocatalytic process, the white porous mat became
purplish violet. As reported in the spectrum of the AuNP / TiO2 hybrid
system, a characteristic absorbance band appeared at about 543 nm, which
corresponded to the surface plasmon resonance (SPR) of the AuNPs (Sun and Xia,
2003). A red shifting and broadening of the absorbance band was observed
with the increase in AuNP size and fiber loading, respectively (data not
shown). The color strictly depends on the size of the nanoparticles
and then their agglomeration at the solid state. According to Bui et al. (2007), such a band-broadening phenomenon is due to the electric
dipole–dipole interactions and coupling occurring between the plasmons of
neighboring particles, whereas nanoparticle agglomeration phenomena
occurred.
UV–vis spectrum of a titania nanofibrous network after gold
decoration (TiO2: 367.8 nm; Au NPs: 543.6 nm).
Due to these features, UV–vis absorption spectroscopy has been used in the
literature as a technique to reveal the changes in the size, shape, and
aggregation of metal nanoparticles in liquid suspension after exposure to
heavy metals as Hg0 (Morris et al., 2002). Both the blue-shifted
wavelength and its extent were proportional to the amount of Hg0 that
entered the liquid suspension. Similarly, when the gold-decorated nanofibers
of titania collected on a quartz slice were exposed to Hg0 vapors
(2 ppm) in air for 15 min, significant blue shifting was reported
(∼ 3 nm; Fig. 4) due to the atomic adsorption of GEM on the
surface. The nanoparticles could be regenerated by heating the sample at
550 ∘C for 3 min, thus removing all the Hg0 adsorbed. Their
recovery was confirmed by the achievement of the original wavelength values
(UV–vis spectra). The regeneration process could be carried out dozens of
times without any noticeable NP deterioration. Similarly, the TiO2
nanofibrous layers coating the metal electrodes of the transducers (Fig. 5)
changed color after photocatalytic treatment (from white to pink).
UV–vis spectra of AuNPs / TiO2 nanofibers before (blue) and after a
15 min exposure to 2 ppm of Hg0 (gray).
The IDE layout (Fig. 5) is comprised of a set of interdigitated electrodes
that
occupies an area approximately 3 × 5 mm, is completely coated with the sensitive
fibers, and has two bonding pads (2 × 2 mm) that will be connected to the
electrometer (DC voltage). Such a planar interdigitated electrode
configuration is most commonly used for conductometric sensing
applications.
The chemosensor fabrication and final structure: IDE dipped (a) and
exposed to UV light (b) for gold decoration.
Figure 6 depicts the current–voltage (I–V) curve of a chemosensor under
a synthetic dry air flow. However, the curve shape was unaltered when air or
nitrogen was flushed over the fibers (Macagnano et al., 2015a), suggesting
that oxygen concentrations poorly affected the electrical properties of such
a chemoresistor. The resistance value of the IDE coated with TiO2 nanofibers before photocatalysis was too high at room
temperature to contribute directly to the final current value. The resulting
linear shape (ohmic behavior) within the selected voltage range (from -3
to +3 V) showed a constant resistance value for the sensor. The very low
value of resistance (∼ 1.2 k) provided the chance to work at low
voltage, with consequent effects on the energy consumption as well as the
lifetime of the material. Moreover, the linearity of the I–V curve let us
suppose that the sensing scaffold had a good adhesion to the metal
electrodes. The electron conductivity has been assumed to occur according
to the percolation model (Macagnano et al, 2017; Müller et al.,
2003), since the titania at room temperature was expected to be an
insulating matrix. When it is metal doped, the electron conductivity is
ruled by thermally activated electron tunneling from one metal island (gold
nanoparticles) to the other. However, the conductivity of the nanocomposite
is lower than that of pure metal (gold) because the electron mean free path
is greatly reduced due to the presence of the dielectric (the titania
crystals). The electrical features, such as the reproducibility of the
fabrication process, of this conductive device have been previously
investigated by the authors (Macagnano et al., 2017, 2015a), showing
encouraging results for the development of a low-cost sensor for mercury
detection. However, in spite of the high sensitivity (LOD: 2 ppt) of the
sensor, a too-long response time was necessary to detect traces of Hg0,
especially if compared to the monitoring instrumentation (Ghaedi et al., 2006;
Sanchez-Rodas et al., 2010; Ferrua et al., 2007) commonly involved in GEM
detection. The long response time was in part due to the
layout of the measuring system, since the sensor was previously housed in a
quartz bottle of 100 mL in volume. Additional time is caused by the
adsorption of Hg0 traces from the surrounding environment (measuring
chamber) up to the achievement of a sufficient number of Hg0 atoms adsorbed on the
surface sensor to be electrically revealed. However, this sensor looks
extremely encouraging if compared to other sensors currently involved in
detecting mercury in air (Drelich et al., 2008; Kabir et al., 2015; Sabri et
al., 2009; Mohibul Kabir et al., 2015; Raffa et al., 2006; James et al.,
2012, 2013; Chemnasiri and Hernandez, 2012; Sabri et al., 2011; Keebaugh et al.,
2007; Crosby, 2013; McNicholas et al., 2011).
The chemosensor current–voltage curve.
Many sensors have been designed and investigated to detect the several forms
of mercury. Most of them have exploited the strong affinity between mercury
and gold (Ford and Pritchard, 1971; Joyner and Roberts, 1973). Several
studies have documented changes in the electrical properties, work function,
and resistance of thin gold films upon exposure to various concentrations of
mercury vapor. For instance, an array of microcantilevers with different
sizes developed by Drelich et al. (2008) could measure different ranges of
mercury concentration (between 37 and 700 µg m-3) and were
capable of revealing up to 10 pg Hg0 adsorbed. However, their sensing
system required both a dust-free gas carrier and a heating procedure (350 ∘C for 20 min) to regenerate the sensors. Gold-based
conductometric sensors, too, have been designed and used to reveal mercury
vapor through their electrical resistance changes (Raffa et al., 2006), but
their sensitivity often seemed poor (about 1 µg m-3). Quartz
crystal microbalance (QCM) devices, too, have been used as Hg0 vapor
sensors (Sabri et al., 2009) due to their high portability, selectivity,
and unnecessary sample pretreatments. Their absorptive capacity was
improved (700 ng cm-2) when the gold electrodes were made rough (more
binding sites). However, additionally to the natural affinity of gold for
mercury, the increased facility for producing and depositing nanoparticles
with noble metals facilitated their use as possible sensors, especially in
aqueous environments (Nolan and Lippard, 2008; Chemnasiri and Hernandez,
2012; Ratner and Mandler, 2015; Dong et al., 2015). James et al. (2012)
developed a highly sensitive chip working in an LSPR mode based on gold
nanoparticles (5 nm diameter) to monitor Hg0 vapors. Such a system
was able to linearly detect mercury concentrations from 1 to 825 µg m-3, but it was strictly related to the flow rate: increasing the
flow velocity (and mass transfer rate) increased the peak shift rate. The
time resolution was limited by the rate of adsorption, which increased with
the
Reynolds number; at the greatest flow rate tested (57 LPM), an ambient
mercury measurement (1 ng m-3) needed 410 h to shift 1 nm, but
accelerating the flow rate could reduce the time resolution.
A homemade measurement chamber to house the chemosensor for
laboratory experiments (left); a plot depicting the transient response curve
to 800 ppb Hg0 (V= 0.3 V).
The normalized sensor response rate to the increasing
concentration of vapor elemental mercury.
Linear fitting parameters of 10 min sensor responses to 21 ppb ≤ [Hg0] ≤ 106 ppb.
ppbv
(Δ I/I0) s-1
SE (±)
R2
21
-7.12602E-10
1.75521E-11
0.86
33
-1.50647E-9
1.05521E-10
0.91
39
-1.78067E-9
1.02615E-10
0.91
40
-1.85901E-9
1.01833E-10
0,92
53
-2.44657E-9
4.24993E-11
0.91
70
-3.19082E-9
2.55882E-11
0.93
106
-4.83599E-9
2.67462E-10
0.88
Gold thin film technology has been recently adopted in a commercial portable
device to detect mercury (Jerome® J405 mercury vapor
analyzer) with a 0.01 µg m-3 resolution and a 750 ± 50 cc min-1
flow rate (http://www.azic.com/jerome/j405/). A series of scrubbers and filtering
systems are able to reduce the effects of interferents on the gold–mercury
interaction. The flow rate and the measuring system layout were key
parameters in the proper working of the device. These features also seem to be
significant for the ES-based chemosensor.
The measuring chamber was designed to reduce the volume (0.7 mL)
and expose the fibers to the gas entry (Fig. 7). Such a measuring layout
was designed to allow the fibrous network to be exposed to the mercury atoms
as delivered into the sensor chamber.
Linear fitting of the normalized sensor response within the first
10 min.
Linear relationships between the normalized response time
and the Hg0 concentration within the range of 20
and 100 ppbv.
Sensing measurements, i.e., the current (or resistance) changes, were carried
out in a continuous mode. The sensor measurements resulted in a change of
the whole current (or resistance, i.e., I =V/R) according to Ohm's law.
Firstly, the sensor was exposed to a flow of Hg0 in air with a
concentration of 800 ppbv for 1 min (Fig. 7, right), and then air was flowed
through the measuring chamber to clean the sensor surface. A rapid decrease
in current was recorded (1.056 × 10-7 A s-1)
when Hg0 entered the measuring chamber. The current curve trend
slightly changed when clean air entered, quickly stabilizing to about the
same current values reached for Hg0 adsorption. Such an effect was
probably due to the Hg0 still housed inside the delivering tubes. The
polluted line contribution was confirmed by further measurements (data not
shown), where the slow current decrease completely disappeared when clean
air never passed through the part of the tubes that had carried Hg vapors. Due to
the strong affinity between Au and Hg0, a 3 min thermal treatment was
necessary to remove mercury from the layer and obtain the same starting current
value.
Figure 8 depicts the normalized sensor response rate, i.e., the normalized
current change per second, toward the increasing concentration of GEM
(ranging between 20 and 160 ppbv). Within this study, the selected flow
rate was kept at 50 sccm in order to avoid turbulence effects. The resulting
logarithmic curve describes the relationship between Hg0 concentration
and the response time; small variations in Hg0 concentration up to 80 ppbv were able to deeply change the response rate.
In contrast, a higher concentration seemed to only weakly affect this sensing feature.
Thus, since a strong relationship is recorded between the concentration and
the response time of mercury under 80 ppbv, it is possible to find a
correlation between the slope of the transient responses within the early
minutes of the sensor response and definite concentrations of Hg0
in air. Figure 9 depicts the linear fitting of 10 min sensor responses when
increasing concentrations of mercury were flowed over the sensor. The related
data are reported in Table 1.
A linear relationship has been reported between the response rate and the
concentration of Hg, according to Eq. (1):
y=-4.56226E-11⋅Hg0,Hg0<100ppb,SE:±1.504E-12,R2=0.99675.
Therefore, when the concentration of Hg increased, the response curve slope
changed too linearly, allowing a limit of detection of about 1 ppbv for
a 10 min exposure (50 sccm). Regarding the main interfering compounds, at room
temperature and in dark conditions the measured current should be due
to AuNPs decorating titania fibers; only chemical compounds interacting with
gold are therefore expected to be mostly responsible for the current changes (i.e.,
halides and sulfides). Thus in a blend of other chemicals, this sensor has
been designed as a relatively selective sensor able to greatly decrease
the environmental disturbances, allowing the investigator or manufacturer to
design and then implement easier strategies to prevent contamination from the
environment (selective filtering systems or coatings). Among the common
potential contaminants, the authors previously investigated water vapor
influence (% RH) and reported no effects on the electrical signals
(Macagnano et al., 2015a).