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Critical Review
A path to impact for autonomous field deployable chemical sensors: A case study of in situ nitrite sensors Tim M. Schierenbeck, and Matthew C Smith Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06171 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017
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Environmental Science & Technology
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A path to impact for autonomous field deployable
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chemical sensors: A case study of in situ nitrite
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sensors
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Tim M. Schierenbeck and Matthew C. Smith*
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School of Freshwater Sciences
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University of Wisconsin - Milwaukee
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600 E Greenfield Avenue,
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Milwaukee, WI, USA.
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KEYWORDS: Sensor, Nitrite, In Situ, Innovation, Field Deployable Sensors, Autonomous
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ABSTRACT
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Natural freshwater systems have been severely affected by excess loading of macronutrients (e.g.,
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nitrogen and phosphorous) from fertilizers, fossil fuels, and human and livestock waste. In the USA,
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impacts to drinking water quality, biogeochemical cycles, and aquatic ecosystems are estimated to cost
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US$210 billion annually. Field-deployable nutrient sensors (FDS) offer potential to support research and
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resource management efforts by acquiring higher resolution data than are currently supported by
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expensive conventional sampling methods. Following nearly forty years of research and development,
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FDS instruments are now starting to penetrate commercial markets. However, instrument uncertainty
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factors (high cost, reliability, accuracy and precision) are key drivers impeding the uptake of FDS by the
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majority of users. Using nitrite sensors as a case study, we review the trends, opportunities, and
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challenges in producing and implementing FDS from a perspective of innovation and impact. We
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characterize the user community and consumer needs, identify trends in research approaches, tabulate
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state-of-the-art examples and specifications, and discuss data life cycle considerations. With further
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development of FDS through prototyping and testing in real-world applications, these tools can deliver
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information for protecting and restoring natural waters, enhancing process control for industrial
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operations and water treatment, and providing novel research insights.
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INTRODUCTION
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Intensifying anthropogenic activities are necessitating an increased environmental monitoring effort
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to obviate water resource-related crises. While analytical chemistry techniques and technologies have
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rapidly improved, the cost and logistics of collecting and analyzing water samples remains prohibitory to
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adequately capture the real-time distribution of contaminants and nutrients in natural, drinking, and
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crop-sustaining waters across meaningful spatial scales1,2. To address this dilemma, a ‘Grand Challenge’
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was posed to the environmental analytical chemistry community to develop capabilities to sample and
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monitor air, water, and soil at higher frequency3. Chemical-sensing field deployable sensors (FDS) are
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systems that operate autonomously in situ and offer the potential to solve the Grand Challenge by
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potentially lowering per-sample and per-measurements costs and offering new methods for remote
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measurements4. Compared to inline cabinet analyzers, test kits, portable and handheld instrumentation,
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FDS offer the additional advantages of dynamic sampling strategies and remote autonomous operation
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that lowers costs associated with manual sampling efforts (e.g., personnel, travel, equipment, sample
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transport)5. In addition, sampling and subsequent analysis on site also removes the risk of error and
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contamination associated with manual sample acquisition, storage, and transport. The portability and
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packaging of FDS enables deployment from a variety of platforms, including buoy systems, remotely
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operated vehicles and gliders, profilers, and underway systems6.
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In 1978 a report for the United States’ National Oceanic and Atmospheric Administration (NOAA)
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surveyed regulatory, academic, and industrial user groups of the marine science community and
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recommended that emphasis should be placed on developing FDS for nutrients along with standardized
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criteria and nomenclature to assess their effectiveness7. Nearly forty years later, the idea of FDS and
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their integration into distributed sensing networks has increased in popularity and scope. FDS have been
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regarded as the ‘the holy grail for environmental analysis’8 because of their potential to provide a
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solution to under-sampling problems in oceanographic research9 and as research tools that will
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revolutionize our understanding of environmental processes10.
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FIELD-DEPLOYABLE SENSING TECHNOLOGY AND INNOVATION
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The appeal for such instrumentation is reflected by a market potential of $150 million by 2020 in the
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United States alone11; however the technologies remain poorly implemented or adopted in
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environmental analytical chemistry12-14. Current technological limitations and prohibitive costs hamper
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FDS inventions from becoming routinely and habitually used at a wide scale15. For FDS that reach high
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technology readiness, the development of functioning, practical, and low-cost FDS remains a
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fundamental yet extremely challenging goal16,17. A significant driving factor that is speeding the progress
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through the lower TRL stages are advances in technologies in rapid prototyping systems such as 3D
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printing which are now being applied to develop inexpensive and integrated fluidic or lab on chip
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systems18. Similarly, open source electronic prototyping systems such as Arduino remove the need to
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develop custom circuitry and therefore enable researchers to quickly develop and test control and
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detection prototype systems with greater speed and reduced cost19. However, one major limitation that
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inhibits the transition of FDS to higher TRL levels and eventual commercialization is the structure of
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funding sources. Systems reaching higher level TRLs require prolonged periods of instrument validation
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in real world monitoring conditions before venture capital or industry are convinced of the investment.
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However, developers of FDS often find themselves in a funding gap where advancement through the
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higher TRL levels requires research that falls outside of the traditional fundamental or applied research,
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which is not viewed as hypothesis driven by many reviewers or funding agencies20. For example, surveys
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by the National Science Foundation in the United States from 2010-2015 showed that higher education
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research and development (R&D) expenditures, were dedicated mainly to basic research activities
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(64.7%) followed by applied research at 26.0% and development at 9.3%. Over the last three years, non-
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federal funding sources have provided the majority of funding for development activities21. More
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recently initiatives such as the Nutrient Challenge led by the Alliance For Coastal Technologies have
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aimed to bridge the high TRL to commercialization gap; however these types of initiatives also assume
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that competitors have existing funding and resources.
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To become truly innovative, FDS must offer economic value and become widely adopted beyond their
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inventors and initial adopters22. This process of transferring technology from research and development
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to commercialization and technological growth and maturity necessarily encounters uncertainties, risks,
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and consequences that must be overcome by developer, practitioner, and sponsor alike. In addition to
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reaching a Technology Readiness Level (TRL)23,24 where uncertainty is low enough for commercial firms
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to invest in the technology and produce it on a mass scale, a user community willing to adopt and
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implement the technology must also be established. As developers improve technology and early
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adopters prove performance in real-world environments, market diffusion is more likely to spread
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across the consumer chasm to reach mainstream consumer markets25. These concepts can be illustrated
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by overlaying elements of technology development given in the TRL scale, with business rule of thumb
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theories such as the S-Curve describing the phases of technological process and Roger’s bell26 curve of
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innovation diffusion driven by consumer adoption behaviors (Figure 1).
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Figure 1. Concepts in the process of technological evolution. Aspects of research and development
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overlaid with a technology adoption bell curve26 that is driven by consumer behavior. The technological
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evolution scale (A) highlights the process involved to take technology from inception to commodity.
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Drivers of this process can be represented at varying scales. The technology readiness level (B) (adapted
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from Mowlem et al.24) lists the stages of technology research and development that can be linked along
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the S-curve of Technological Process23 (C) (solid line) and coupled to the process of innovation (dashed
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lines). This comprises the entire technology life cycle beginning with decision to undertake research
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followed by stages of development and commercialization along with market adoption and diffusion (D).
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This review uses nitrite as a case study for the development and implementation of FDS in aquatic
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environments, highlights progress towards achieving answers for the Grand Challenge and addresses the
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requirements and challenges ahead. While some optical probes have experienced full automation and a
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level of success in situ, electrode and biochip-based sensors still require reaching relevant limits of
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detection along with rugged interfaces to support long-term unattended deployments in natural
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waters27. Nitrite has been selected as a parameter for evaluation of FDS because of its significance in
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aquatic environments and consequently its importance to environmental researchers and resource
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managers (Figure 2). We assess detection systems and state of the art instrumentation for nitrite as a
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proxy for other optical and electrochemical sensors, as the anion is subject to both forms of detection
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and therefore provides a relatively straightforward analysis opportunity with a broad appeal.
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SIGNIFICANCE OF NITRITE IN AQUATIC ENVIRONMENTS
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Intensifying human activities (energy production, crop cultivation of legumes and rice, fertilizer and
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feed applications, human and animal waste, food preservatives) have more than doubled the amount of
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bioavailable nitrogen in the environment and increased the amount of nitrate and nitrite entering
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aquatic environments and water supplies28. Subject to abiotic and biotic transformation through
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photochemical degradation, nitrification, denitrification, and annamox processes, nitrite is an
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intermediate species in the overall nitrogen cycle that if accumulated can have significant negative
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impacts at global scales for human, animal, ecosystem, and economic health. Nitrite can be responsible
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for methemoglobinemia, a condition that reduces the oxygen carrying capacity of red blood cells. This
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potentially fatal condition, along with nitrite’s role as a suspected carcinogen29,30 and indicator of fecal
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pollution, have warranted legal recognition of nitrates and nitrites through thresholds in drinking water
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set by institutions such as the World Health Organization (WHO) and the United States Environmental
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Protection Agency (EPA) (Table 1). The risk for aquatic animals is even greater because of nitrite
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exposure at the gill membrane31 and necessitates continuous nitrite level regulation and monitoring in
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aquaculture operations (Table 1) especially in recirculating systems32.
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Table 1. Relevant ranges of nitrite in aquatic environments. Matrix Metric Nitrite Concentration
Drinking Water
Reference
EPA National Standard
1 mg/L NO2-N (71.42 μM)
WHO Standard
3 mg/L NO2 (65.20 μM)
European Union Water Directive Standard
0.5 mg/L NO2 (10.87 μM) Consumer tap
Geographic Area
Approximate Range
Surface Ocean
0.1 - 200 nM
Deep Ocean
0.1 - 5 nM
Estuarine
0.5 - 1.5 μM
Coastal
0.1 - 2 μM
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-
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Natural Waters
Median 96 h LC50 (Adjusted for 20 ppm Chloride)
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0.1 mg/L
NO2
(2.17 μM) Treatment plant Lab Resolution
0.1 nM
0.01 μM
Coldwater Species
4.6 - 9.4 mg/L NO2-N (0.33 - 0.67 mM)
Warmwater Species
6.4 - 144 mg/L NO2-N (0.46 - 10.28 mM)
Other Families
9 < x
