Preliminary Analysis on Cellulose-based Gas Sensor by Means of

Aerosol Jet Printing and Photonic Sintering

Edoardo Cantù

, Matteo Soprani

1,2

, Andrea Ponzoni

1,2

, Emilio Sardini

and Mauro Serpelloni

Department of Information Engineering, University of Brescia, Via Branze, Brescia, Italy

National Institute of Optics, National Research Council, Brescia, Italy

Keywords: 3-D Printing, Aerosol Jet Printing, Photonic Sintering, Ammines Sensing, Cellulose-based Sensor.

Abstract: In this paper, we present a preliminary analysis on the possibility to realize low-cost and eco-friendly

cellulose-based gas sensors by means of Aerosol Jet Printing (AJP) and flash lamp annealing (FLA). To the

authors knowledge, it is the first time that these two techniques are combined in the realization of such a

device. The intrinsic hygroscopic properties are the key element of this device: cellulose contains substantial

amount of moisture, adsorbed from the environment, enabling the use of wet chemical methods for sensing

without manually adding water to the substrate. The sensors were tested in terms of electrical resistance. The

penetration of the carbon ink in the cellulose network was stated thanks to cross-sections captured at the

microscope. Once placed in a damp environment, all the sensors showed a comparable behavior settling at an

asymptotic value of 3.68 MΩ (relative standard deviation of 8%). In presence of different concentration of

NH4OH, the sensors showed a resistance proportional to the amount of analyte present in the working volume,

showing 25.6% increase compared to the 0.5 M concentration, while 34.1% compared to the 1M.

1 INTRODUCTION

Printed electronics represents an attractive and

innovative field of application of Additive

Manufacturing (AM) with flexible electronic devices,

paper electronics and wearable devices that are

becoming widespread coming with the need for

advanced printing processes, reducing the number of

processing steps, material waste and production cost

(Gandhiraman et al., 2014).

3D printing is considered a valid candidate for the

new generation of biosensors (Ragones et al., 2015;

Yang et al., 2016), and Aerosol Jet Printing (AJP) is

belonging to this technological family. AJP is a

direct-write printing technique which deposits a

continuous aerosol beam of liquid droplets,

generating specific surface features without

masks(Hoey et al., 2012; Lee, An and Chua, 2017).

The liquid inks typically have a viscosity between 1

and 1000 cP, in case of pneumatic atomization, and 1

to 5 cP, in case of ultrasonic atomization (Tan, Tran

and Chua, 2016). Once the mist is generated, it passes

through the virtual impactor to regulate droplets

dimensions and then it is focused on the substrate

thanks to a coaxial sheath gas flow(Binder, Glatthaar

and Rädlein, 2014; Wadhwa, 2015; Optomec, 2017).

This 3D printing technique is adopted in different

fields: high-efficiency solar and fuel cells, thin-film

transistors, resistors, antennae, MEMS,

photodetectors, thermistors, etc. But it is also used in

applications for the biological field like high-density

assays for drug discovery, lab-on-chip devices,

protein and glucose sensing (OPTOMEC, no date;

Coupland et al., 2010; Yang et al., 2016; Wang et al.,

2017; Bolse et al., 2017; Liu et al., 2017; Agarwala

et al., 2018; Baldwin et al., 2018; Cantù et al., 2018;

Gupta et al., 2018; Khorramdel, Torkkeli and

Mäntysalo, 2018; Di Novo et al., 2019).

In the printed electronics field, the sintering step

is mandatory to ensure a proper conductivity in the

printed layer, which is usually performed by means of

hoven for high temperature processes. One of the

emerging topics in printed electronics is the usage of

paper due to its unique capabilities (high Young’s

modulus, biodegradability, biocompatibility,

renewability, low-cost, lightweight) (Kim et al.,

2014). This material needs great attention in sintering

printed inks due to low melting/ignition point.

Therefore, only low-temperature processes are

allowed, which do not ensure high conductivities.

Photonic sintering, also known as flash lamp

annealing (FLA) or intense pulsed light (IPL), is a

200

Cantù, E., Soprani, M., Ponzoni, A., Sardini, E. and Serpelloni, M.

Preliminary Analysis on Cellulose-based Gas Sensor by Means of Aerosol Jet Printing and Photonic Sintering.

DOI: 10.5220/0009095702000206

In Proceedings of the 13th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2020) - Volume 1: BIODEVICES, pages 200-206

ISBN: 978-989-758-398-8; ISSN: 2184-4305

technology, developed by Novacentrix, in which the

heating process is accomplished thanks to a couple of

xenon flash lamps and mirrors which radiate energy

toward the sample that must be sintered, following

phenomenon of melting/sintering point depression in

nanoparticles described by the Gibbs-Thomson

equation.

The advantages are remarkable: this is a fast

sintering technique (heat treatment in the

milliseconds scale), it produces minimal damage to

low temperature substrates and avoids unwanted

processes (diffusion). Possible applications range

from thin-film transistors, solar cells, RFID

components to wearable devices, using conductive,

bioresorbable or ceramic inks (silicon, silver, zinc) on

different kind of substrates (wood, cardboard,

textiles, glass, kapton (West et al., no date; Akbari et

al., 2017; Mahajan et al., 2017; Mudgal et al., 2017;

Rizwan et al., 2017; Bobinger et al., 2018; Cronin et

al., 2018; Danaei et al., 2018; Li et al., 2018; Schube

et al., 2018).

In light of this, the present work aims to propose

the combined use of AJP and FLA, to realize a

ellulose gas sensor, designed for food industry. Food

waste is one of the hardest challenges for humanity as

it impacts on society not only in food terms (direct

food loss and waste), but also in environmental terms

(exploitation of water and land, disposal, emissions)

(Springmann et al., 2018) For example, the use by-

date is an estimation of the date on which a perishable

product may no longer be edible, and this number

cannot clearly state the condition of the food (Labuza

and Taoukis, 1990).

Ammonia and its compounds can be considered

toxic, capable of generating damages and unwanted

pathologies to human body if ingested (nausea,

migraine, vomiting, allergies) (Douny et al., 2019;

Filipe-Ribeiro et al., 2019). The research is working

on wireless and automated systems capable to

continuously give a feedback of the sample (Bellitti

et al., 2017; Tonello et al., 2017). The possibility to

include a sensor inside the packaging appears to be a

solution which can monitor the current state of the

food defining its freshness (Yam, 2012; Liu et al.,

2015).

Paper-based sensor with carbon electrodes

exploits the intrinsic hygroscopic characteristics of

cellulose paper. Cellulose fibers contain a certain

amount of moisture, even if paper looks dry. This

aspect is mandatory in evaluating the presence of

water-soluble gases. In presence of a water-soluble

gas (due to food degradation), electrical impedance or

conductance of paper can be probed thanks to carbon

electrodes realized on the surface of the paper sheet.

The feasibility of these sensors has been recently

reported in literature using a low-cost cutter plotter

for preparation (Barandun et al., 2019). Our goal is to

develop this class of devices by combining the AJP

and FLA methods, investigating the improvement we

can achieve thanks to the more sophisticated

preparation technique. Chapter two will present

methods followed during tests, while chapter three

shows the results obtained during validation and

characterization tests. Chapter four will summarize

the preliminary results achieved, with future

perspective of this application.

2 SENSOR FABRICATION

2.1 Sensor Design

Optomec’s Aerosol Jet Printing AJ300 system was

employed in the fabrication of the proposed gas

sensor: this 3D printing machine is designed to

interface with AutoCAD drawings. Specific attention

was put in the definition of printing parameters,

considering inks viscosity, in order to achieve a

homogeneous filling of the interdigitated geometry,

also thanks to the use of a crossed pattern. The sensors

were designed thanks to AutoCAD software

(Autodesk), a devoted Optomec’s gadget for

AutoCAD ensured a proper ink filling of closed

shapes. The lines were designed as polylines to

improve the timing and the homogeneity of printed

lines.

Surface area of the interdigitated electrodes and

spacing in between, are the two main factors that must

be considered during the design phase, thus

determining the overall impedance of the sensors,

which have a critical effect on its sensitivity. More

specifically, a total sensing area of 150 mm

and a

spacing of about 1.5 mm were selected (fig. 1).

Figure 1: AutoCAD layout of the sensor.

Preliminary Analysis on Cellulose-based Gas Sensor by Means of Aerosol Jet Printing and Photonic Sintering

201

2.2 Materials and Methods

Whatman

chromatography 1 cellulose paper is the

selected substrate on which we printed our sensors,

due to the capability of this kind of paper to capillary

handling liquid samples. The second reason we

employed this material is the high degree of purity

respect to other commercial grades which include

chemical additives that could influence the

functioning principle of the proposed sensors. Thanks

to its highly hygroscopic properties, cellulose fibers

within paper contain a substantial amount of

moisture, even if it looks dry (at a relative humidity

of 50%, paper contains ~5% water by weight).

Additional ions coming from the dissociation of

water-soluble gases on the surface of paper increases

its ionic conductance.

Carbon ink (EXP 2652-28) was purchased by

Creative Materials Inc., characterized by a starting

viscosity of 15-20 mPa∙s. The ink was properly

atomized together with its own thinner to obtain a

better printability and adhesion to the substrate.

Carbon ink specific process parameters are reported

in Table 1, while figure 2 presents a prototype and our

AJ300 during printing.

Table 1: Printing process parameters for C ink.

Process Parameters

Sheath

as flow

(

SCCM

)

110

Atomizer flow

(

SCCM

)

770

Exhaust flow (SCCM) 750

Process speed (mms-1) 4

Plate temperature (°C) 70

Figure 2: AJ printer printing a first concept (left); a printed

sensor in its final layout (right).

With the aim to define the sintering parameters

for our carbon sensors, a sample geometry was

considered in order to define FLA sintering

parameters. These samples were tested in terms of

electrical resistance, evaluated thorough the digital

bench-top multimeter Hewlett-Packard 34401a,

applying testing probes to the extremities of each

path, in standardized and repeatable points, thus

measuring the resistance offered by all its length.

An optical microscope by Orma Scientific

NB50T (trinocular zoom 0.8x–5x–LED), with its

devoted software and HDMI MDH5 camera model,

was used to acquire the images and to evaluate the

features of the printed elements (Orma Scientific,

Sesto San Giovanni, Milan, Italy).

After the definition of the abovementioned

parameters, four sensors were put in four different

vials to examine their capability to absorb ions

present in a humidified atmosphere and their response

was simultaneously studied in terms of resistance. 5

ml was the selected working volume and distilled

water was employed for the stabilization step (used to

reach a relative humidity of 100 % inside the vial to

let sensors work properly in presence of water-soluble

gases), aiming to state the repeatability of this kind of

measurement principle with these AJ printed sensors.

After this step, a qualitative study was performed by

means of distilled water/NH

OH solutions in two

different concentrations, 1 M and 0.5 M, which will

develop a gas-phase of ammonia in a humidity

saturated air background. Additional tests have also

been carried out with ethanol in an ambient air

background to test the working mechanism of this

class of devices. Four digital bench-top multimeter

Hewlett-Packard 34401a were connected thanks to a

GPIB-USB cable to a personal computer and the

behaviour of the sensors monitored thanks to a

program written in LabVIEW.

3 PRELIMINARY RESULTS

Once printed, carbon samples must be sintered in

order to ensure the realization of a unique conductive

track. Impulse duration was studied keeping it fixed,

initially at 1000 μs, increasing the lamp voltage from

200 up to 300 V. The same approach was replicated

for other three pulse time-lengths, specifically 1250,

1500 and 1750 μs. At the end of this step the best

sintering parameters which gave the lowest resistance

were selected. As evidenced in figure 3, 1000 μs

seems to be a too short impulse duration, being not

able to completely penetrate the whole thickness of

our chromatographic paper (paper thickness is about

180 μm). 1250 and 1500 μs showed a similar

behavior, with the second one presenting a lower

resistance.1750 μs and 250 V are the best sintering

parameters, as they allow to reach a final resistance

of 2.8 kΩ (relative standard deviation of 7%). So, the

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complete pool of production parameters for our

sensors are the ones reported in table 1, with six

consecutive depositions, sintered at 250 V, 1750 μs.

Figure 4 displays a magnification of an

interdigitated printed element, suggesting the need of

a proper wavelength to fully penetrate the cellulose

sheet to sinter carbon ink. The effectiveness of the

deposition process was initially tested evaluating the

resistance of individual dry electrodes resistance,

after both printing and sintering steps. It was found a

mean value of 8.45 kΩ (relative standard deviation of

23%) after printing, while 7.44 kΩ (relative standard

deviation of 21%) after sintering. It is interesting to

compare these values with those reported in literature

for a preparation method based on an ultra-low cost

cutter plotter printer. In this case the average

resistance is of an individual electrode is 6.40 kΩ with

a relative standard deviation of 43% (Barandun et al.,

2019). The presented results demonstrate a difference

in standard deviation of 22% so the combine use of

AJP and FLA have improved the sensors realization

procedure.

Figure 3: Resistance measurements as funciton of voltage

at different impulse duration, for C samples.

Figure 4: Magnification of an interdigitated element

(bottom).

To validate and demonstrate the functioning of

our paper-based sensors, they were tested in a damp

environment. Four sensors were simultaneously put

in four different vials in presence of a working

volume of 5 ml of distilled water. Figure 5 shows the

behaviour of the sensors reaching the stabilization at

an asymptotic value of

3.68 MΩ

(relative standard

deviation of 8%). The condition step was 30 hours

long in order to clearly evidence the stabilization of

the sensors reaching a relative humidity inside the

vial of 100 %. Since sensors resistance appeared to be

stable before reaching the thirtieth hour, in future

works the precondition duration will be performed

taking into account only the strictly necessary time.

Figure 5: Resistance values during stabilization of four

sensors in distilled water.

After this, a simple qualitative study was

performed aiming to state the capability of the sensor

to recognize different concentrations of the same

analyte in a humidity saturated background, which

optimize the sensing mechanism of this kind of

devices (Barandun et al., 2019). Once run a

precondition phase in presence of 5 ml of water, one

sensor was exposed to the vapours generated by a 1M

solution of NH

OH in water, one sensor with a 0.5M

solution of NH

OH and one with a new vial with

distilled water (blank sample). To further test the

effect of the working environment, an additional

experiment has been carried with a fourth sensor

exposed to vapors generated from an ethanol solution.

This allows to test the effects of extreme working

conditions (saturated ethanol, vapors).

Figure 6 evidences these experiments.

The data regarding NH

OH vials are reported as

ΔG/G

in percentage in table 2. ΔG values are

calculated as the difference between the maximum

and minimum resistance values when the analyte is

recognized after vial change, while G

is the

minimum resistance value in the same time interval.

The sensors show a 25.6% increase compared to the

0.5 M concentration, while 34.1% compared to the

1M. Thus, with an 8.5% difference between the

highest and the lowest concentration.

The exposure to saturated ethanol vapours in the

ambient air background cause a sudden and large

increase of the sensor resistance. This extreme

behaviour is likely to arise from the large ethanol

concentration (saturated vapours), which modifies the

condition of the cellulose fibres making them

Preliminary Analysis on Cellulose-based Gas Sensor by Means of Aerosol Jet Printing and Photonic Sintering

203

anhydrous, hence much less suitable for ionic

conduction. The result further supports the working

mechanism proposed in literature, based on the

intrinsic ionic conductivity of the water layer

covering cellulose in ambient conditions. To better

clarify this point, we have planned additional tests

with lower ethanol concentrations.

Table 2: ΔG comparison between 0.5M and 1M solutions.

Concentration (M) ΔG/G

(%)

0.5 25.6

1 34.1

Figure 6: Behaviour of the sensors in presence of different

analytes (water, ethanol, NH

OH 1 M and 0.5 M).

4 CONCLUSIONS

We present a new kind of disposable gas sensor,

produced thanks to Aerosol Jet Printing and Flash

Lamp Annealing on a cellulose-based material. The

production method presents good reproducibility

despite a porous substrate such as chromatographic

paper. In this first research step, we want to verify the

functioning of the sensors, considering a first design

which is not optimized for our future perspectives.

The following steps will regard the complete

optimization of the production procedure, thus

redefining the optimal sintering parameters for the

new the design of the sensors. Once tested in presence

of a damp environment, our device behaved in the

same way, showing an asymptotic value of

3.68 MΩ

regardless of the specific starting conducting value.

The sensors were also qualitative tested, showing a

signal proportional to the amount of NH

OH present

in the sample volume. Future developments will

regard the investigation of the limit of detection of

these cellulose-based sensors and their selectivity. In

particular, tests are planned to check the effects of

ethanol vapours, which are likely to alter the hydrous

status of the cellulose fibres, especially at high

concentrations. More in general, considering the

prospective to tune the sensing properties of this class

of devices to a given gas-target, this possibility should

be strictly checked against its working principle,

whose simplicity represents at the same time the

strength and the weakness of these devices. Indeed, if

gases containing ammine groups can be easily sensed

by this simple and extremely cost-effective device,

any alteration of the hydrous status if the paper may

deeply alter its sensing capability. Any

functionalization of the paper aimed to tune its

sensitivity to a given gas-compound, should be

carried out in careful consideration of this aspect.

ACKNOWLEDGEMENTS

The authors declared no conflict of interest.

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