A Wireless Sensor Network based on Laser-annealed ZnO

Nanostructures for Advance Monitoring in Precise Agriculture

Davide Polese

, Francesco Maita

, Ivano Lucarini

, Antonio Ferraro

2,3

, Antonio De Luca

2,3 c

Domenico Cannatà

and Luca Maiolo

IMM - Istituto per la Microelettronica e Microsistemi, CNR- Consiglio Nazionale delle Ricerche,

Via del Fosso del Cavaliere n.100, 00133 Roma, Italy

Department of Physics, University of Calabria, 87036 Rende, Italy

CNR-Licryl Lab., NANOTEC Institute, 87036 Rende, Italy

Keywords: Wireless Sensor Network, ZnO Nanorods, Laser-annealed Gas Sensors, Precise Agriculture.

Abstract: Plants own a complex way to communicate with each other based on the exchange of chemical and electrical

signals. Indeed, plants are capable of creating extensive communication networks thus warning each other of

the presence of pests. In response, plants trigger natural strategy against the infestation. The main tool used

by plants for exchanging information is the emission and detection of specific volatile organic compounds in

air. To this end, monitoring these compounds can be crucial to reveal the state of health of a cultivation far

before visual symptoms arise. In this work, we present a wireless sensor network where each node is based

on highly sensitive zinc oxide nanostructures enabling the detection and the discrimination of several chemical

gases such as CO, CO

, NO, NO

, CH

, etc. The response of each sensor is tuned by using excimer laser

annealing procedure, a technique that changes the electrical and morphological properties of the sensing

material. This wireless sensor network can be an appealing solution to capture signals coming from the plants

without the usage of bulky and expensive equipment.

1 INTRODUCTION

In modern agriculture, the monitoring of the health

state of crops is becoming of primary relevance to

improve the productivity and reduce both

environmental and economic costs. Indeed,

preservation of crops in the optimum conditions

allows enhancing the production, but in particular

means recognizing the first symptoms of any plant

disease thus avoiding the broadening of the infection

to other part of the crop. A quick intervention allows

reducing the cost of phytosanitary intervention and

the repercussion on the environment.

From the birth of the farming, the farming fight

against crop failure is the main issue, but nevertheless

just from the middle of the seventeenth century a

scientific approach was applied to the detection of

https://orcid.org/0000-0002-6332-5051

https://orcid.org/0000-0002-0822-2850

https://orcid.org/0000-0003-2428-9075

https://orcid.org/0000-0003-4072-0099

https://orcid.org/0000-0003-3220-5353

plant diseases (Martinelli et al., 2015). From the

beginning, the visual inspection and the visual

symptoms have been the main approaches of

diagnosis, but with the technological improvement

new methods coming out for early stage diagnostics.

In particular, plants emit in surrounding

environments several volatile organic compounds,

that are connected with several plant functions such

as: growth, communication, lack of nutrients, defence

and survival (Baldwin, Halitschke, Paschold, Von

Dahl, & Preston, 2006). In particular, appearance or

changing in concentration of VOCs are correlated

with plant stress or incoming disease and they

represent one the most promising markers for early

detection (Martinelli et al., 2015).

Besides VOCs, also gaseous pollutant presence

affects the plant metabolism and it can be used as

Polese, D., Maita, F., Lucarini, I., Ferraro, A., De Luca, A., Cannatà, D. and Maiolo, L.

A Wireless Sensor Network based on Laser-annealed ZnO Nanostructures for Advance Monitoring in Precise Agriculture.

DOI: 10.5220/0009368201770181

In Proceedings of the 9th International Conference on Sensor Networks (SENSORNETS 2020), pages 177-181

ISBN: 978-989-758-403-9; ISSN: 2184-4380

177

measurement of the plant stress level. In addition to

that is one of the fundamental element of the

photosynthesis, NO

presence has many negative

effects on the plant metabolism (Kozioł & Whatley,

2016) as well as CO concentration (Zimmerman,

Crocker, & Hitchcock, 1933).

Up to now, scholars have obtained the estimation

of the VOCs or gas pollutant concentration by using

classical laboratory instruments such as gas

chromatography mass spectroscopy. This kind of

instruments are bulky and not appropriate for a

continue monitoring in field. To this purpose, in the

last year, approaches based on electronic noses have

been proposed (Gardner & Bartlett, 1994;

Laothawornkitkul et al., 2008). Nevertheless, until

now, these approaches are based on single instrument

and they are not implemented for a continuous

monitoring. Moreover, a plethora of low-cost and

high sensitive sensors based on different transduction

methods are starting to be proposed as valuable tools

to detect specific VOCs and biomolecules. These

devices can be assembled combining together

commercial sensors (Leccese et al., 2017) or

exploiting innovative materials and blend of

nanostructures (Kaushal & Wani, 2017; Palneedi et

al., 2018) even on flexible substrates (Maiolo et al.,

2013; Zampetti et al., 2011).

In this work, we present a wireless sensor network

endowed of several sensors, both physical and

chemical for the continuous monitoring of the

environment of the crop. In particular, a set of

gaseous pollutants can be efficiently detected and

discriminated such as CO, CO

, CH

, NO as well as

. The paper is organized as follow: in section 2,

the materials composing the wireless sensor node are

described regarding both electronics and sensors, in

section 3, some results of sensor tests are shown and

finally section 4 presents the conclusions.

2 MATERIALS AND METHODS

2.1 Gas Sensor Fabrication

Sensing materials based on nanostructures allows

improving and tuning the sensor characteristics

(Cuscunà et al., 2012; Galstyan, Comini, Baratto,

Faglia, & Sberveglieri, 2015; Polese et al., 2017,

2015; Rickerby & Skouloudis, 2016). In this work,

we propose the use of laser annealed ZnO

nanostructures as active sensing material.

To fabricate the nanostructured gas sensors

proposed, we adopted a standard interdigitated

structure where the sensing layer acts as a bridge to

create a variable resistance. To this purpose, a 3-inch

silicon wafer with a layer of thermal silicon dioxide

1 μm thick was used as device substrate. After a

surface cleaning procedure in an oxidizing solution of

and H

(7:4) and HF bath, we deposited a

precursor layer of commercial ZnO colloids by spin-

coating technique. Subsequently, we grew ZnO

nanorods (NRs) by using a recipe explained in previous

works and we lithographically defined ZnO NRs

islands (Fiaschi et al., 2018).We chose to adopt

hydrothermal technique to grow ZnO nanorods since it

represents the simplest and the cheapest way to obtain

ZnO nanostructures (See Fig. 1) (Fiaschi et al., 2018).

This procedure provides disordered ensemble of

nanorods with a good crystallinity (e.g. standard

wurtzite structure) and a high conductivity under UV

irradiation (in the range of 10 Ohm/cm in case of

lasered samples) (not shown in this paper) (Polese et

al., 2019). After the growth of ZnO NRs, we irradiated

the sample at two different energy densities (75 and

100 mJ/cm

) exploiting a XeCl excimer laser

(308 nm), thus obtaining a partially melting of the

nanorods tips in the first case and a fully melting of the

structures in the second one. The irradiation procedure

is provided by lasing five shots on the same point

through a beam with a rectangular shape of 1x70 mm.

Figure 1: A SEM image of the ZnO NRs grown by

hydrothermal technique.

Finally, we patterned interdigitated metal

structures on the ZnO NRs islands, cut the samples

and glued them onto a PCB. This fabrication flow

chart has been adopted to ensure a uniform laser

annealing on the material without changing locally

the thermal conductivity related to the presence of

underlying metal stripes.

WSN4PA 2020 - Special Session on Wireless Sensor Networks for Precise Agriculture

178

2.2 Material of the Node

A central unit based on three gas chemical sensors as

well as complementary sensors (temperature and

humidity) composes each node. The single sensor

node has been fabricated on a standard oxidized

wafer, then cut and glued on a small PCB.

Interconnections are made with wire bonder machine

by using a Kulicke and Soffa model 4123. The three

gas sensors are based on ZnO nanostructures

annealed through an excimer laser irradiation at

different energy density to obtain a specific material

conductivity and morphology (see Fig.2). Two

commercial sensors have been assembled in the board

to collect data about temperature and humidity. In

particular, we install a SHTC3 by Sensirion (that

measures both temperature and relative humidity).

Figure 2: Three sensors of ZnO NRs laser-annealed at

different energy densities (0, 75 and 100 mJ/cm

) mounted

on their respective PCB.

2.3 Node Electronics and WSN

Architecture

With the intent of maximizing the performance of the

node, we considered a set of functions that need to be

properly designed: i) sensor data acquisition, ii) data

communication and energy management and

harvesting. To this purpose, the node is equipped with

electronic interfaces for the custom sensors, a System

on Chip (SoC) composed of a microcontroller and a

RF interface, a Battery Management (BM) and a

Maximum Power Point Tracking (MPPT) circuit for

the power management.

Figure 3: Schematic representation of the node architecture.

In the figure, the main parts of the node are highlighted.

A schematic representation of the node structure

is sketched in figure 3. With more details, the Texas

Instrument CC1352 SoC has been selected for the

application. It is composed of an Arm® Cortex®-

M4F microcontroller (µC) and a multiprotocol RF

interface. The µC can acquire the data coming from

the commercial physical sensors (temperature) by

means the standard communication digital ports and

the data from the chemical gas sensors (RH and

volatile pollutant) digitizing the analog value coming

from the electronic sensor interfaces. The RF

interfaces support several 2.4 GHz protocols and also

sub-GHz long range protocols. In particular, our SoC

supports the Thread network protocol

(https://www.threadgroup.org) that allows easily

developing mesh networks. Finally, the node has been

equipped of a flexible photovoltaic panel (SP3-37 by

PowerFilm Solar) and a battery for the energy supply.

To optimize the energy consumption, harvesting and

storage, a MPPT and a BM circuits have been

integrated. They should guarantee a continuous

functioning of the nodes in the long period without

further maintenance.

Each node has been equipped with a multiband

RF interface in order to have both a standard interface

at 2.4 GHz for the local data request and, sub-GHz for

long range communications. Even if the network is

based on sub-GHz carrier, that allows a longer

distance of transmission, each node maintains the

possibility of being locally interrogated by mean

standard Bluetooth interface. This possibility has

been maintained for three main reasons: i) guarantee

the possibility of accessing to the node status (sensors

and node status) directly on field with a general-

purpose device as a smartphone or tablet; ii) allows

the download the data from isolated nodes, iii)

transfer the position information from a mobile GPS

A Wireless Sensor Network based on Laser-annealed ZnO Nanostructures for Advance Monitoring in Precise Agriculture

179

module during the installation procedures. On the

other hand, the sub-GHz carrier frequency is useful

for long distance communications. Considering the

low data rate, due to the slow dynamics of the

physical and chemical changes in precise agriculture

applications, these quantity can be monitored

reaching even distance as long as 1 km. This distance

are several order of magnitude larger of the crop

information detail that would be reached.

Since each node of the WSN would be placed in

the transmission area covered by several other nodes,

the characteristics of network reconfiguration of the

Thread protocol will be deeply investigated to extend

the working period of the network.

3 RESULTS AND DISCUSSION

To evaluate the properties of the ZnO nanostructured

sensors in detecting low concentrations of pollutants

(in the range of tens of ppm) we preliminary tested

the device response of each gas in a controlled

environment. In particular, we used a customized

sealed stain steel chamber with a controlled inlet and

outlet to evaluate the sensor response at room

temperature. We adopted dry air as carrier gas and we

measured the output of the devices under a UV

illumination at a frequency of 365 nm and a power

density of about 16 µW/mm

. We used three different

devices with a sensing material based on as deposited

ZnO nanorods and two recrystallized structures by

using an excimer laser annealing at 75 and 100

mJ/cm

. In figure 4 a score plot shows how it is

possible to discriminate all the five analytes (CO,

, NO, NO

, CH

) proposed as testing analytes. In

this case, we report the sensor response for a gas

concentration of 150 ppm. Projection of the data onto

principal component plane has also the advantage of

allowing the implementation of calibration

algorithms among different nodes (Marco &

Gutiérrez-gálvez, 2012; Polese et al., 2013; Yan &

Zhang, 2015).

In order to estimate the node working life, the

power consumption and the energy recharged by the

solar panels have to be evaluated. It is important to

note that the quantity under monitoring changes

slowly during the day, so, a sampling rate every

minute is more than enough. Considering a data rate

of 250 kbit/s the transmission time is limited to lesser

than 1 ms to transmit all the sensors data. In this case

the power consumption of the SoC can be limited to

less than 100 µW. For the sensors, the main

consumption is due to the UV leds that can be

estimated in less than 25 mW. On the other hand, the

selected photovoltaic panel can generate up to 70 mW

at standard radiation. In these conditions, a long-term

operating time could be obtained.

Figure 4: Scores plot of the three ZnO nanostructured

sensors. As can be seen, the five analytes measured in the

experiments can be easily discriminated.

4 CONCLUSIONS

Plants are organisms capable to communicate through

a large and complex network of bioelectrical and

chemical signals. The possibility to capture this secret

language is the key to control and protect cultivation

in precise agriculture scenario. A wireless sensor

network composed by low cost and highly sensitive

devices can be useful to collect signals coming from

plants to preventively respond in case of infestation

of external harmful stimuli. We proposed a wireless

sensor network based on nanostructured zinc oxide

sensing material to detect pollutants and VOCs and

discriminate these signals in air at room temperature

to trigger a proper action in case of starting threat. We

adopt excimer laser annealing as unique technique to

tune the properties of the sensing materials. We

believe that these devices can pave the way to the

manufacturing of low-cost sensing systems to be

deployed in large cultivation in both greenhouses and

open field.

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