Nutrient Detection Sensors in Seawater Based on ISI Web of Science Database

2.1. Database Compilation

We build this database using the following topics searched in core database of Web of Science (1985-2020): “sensor” and “seawater or sea water or saline water or marine water or salt water or ocean” and “nutrient or nitrate or nitrite or phosphate or ammonia or silicate” in July 2020. Here, for supplying more valuable information for the development of nutrient sensors applied in seawater, we just collected literatures focusing on the technological development of sensors, excluded those focusing on application. In total, 42.21% of the screened literatures were published during the last five years (2016-2020). For each literature, we extracted the information: tested analytes, detection method of sensors, LOD, and publication year. All tested analytes were categorized into phosphate, nitrite, nitrate, ammonium, and silicate. As for the detection methods, we included spectrophotometry (ultraviolet spectrophotometry, visible light spectrophotometry), fluorimetry, colorimetry (visual colorimetry, photoelectric colorimetry), chromatography, microfiber, microfluidics, electrochemistry, optofluidics, and carbon nanotube. We unified all the units of LODs as μM (μmol/L).

2.2. Data Analysis

We performed one-way ANOVA to test the differences of LODs among different detection methods within one analyte. All data were checked for normality before conducting the ANOVA tests and were log-transformed to meet normality and homogeneity assumptions [19]. If significant effects are present in the ANOVA, then Tukey’s test was used for post hoc analysis of significant differences among detection methods. All statistical analyses were performed by SPSS statistics software (IBM, 20.0).

3.1. Research Status of Nutrient Sensors in Seawater

To a certain extent, quantity of literatures could reflect scientific research status in this area. After screening from 538 literatures searched in the core database of Web of Science, we finally collected 109 literatures focusing on the technological development of nutrient sensors applied in seawater. Table 1 shows the summary of sensors for the nutrient detection in seawater. Here, we classified analytes mainly by phosphate, nitrate, ammonium, and silicate related to different detection methods with LOD and publication date.

From Table 2, we can see that the quantity of literatures has increased with the development of decades. In particular, during 2011-2020 in both analytes and detection methods, there are more literatures focusing on the technological development of sensors. Furthermore, this trend also means that more literatures may appear during the next ten years to meet the needs from all walks of life and from many different areas. Actually, nitrate determination was firstly taken by electrochemical methods in 1834 [117]. It has been attracting more attention in the world today from different perspectives. A review published in 2021 summarizes the advances in knowledge in terms of the modes of action of devices and deployment strategies, identifying the current limitations and future challenges for the electrochemical detection of nutrients in marine environments [118]. The application of electrochemical sensors including potentiometric, voltammetric, and field-effect transistor sensors for nitrate, nitrite, ammonium, and phosphate determination in aqueous environments was reviewed [16]. The recent advances in ISE sensing platforms for environmental water analysis from on board to in situ approaches were also reviewed [119].

In terms of quantity, the above obvious result may mainly attribute to (1) emerging technologies. In 2003, Thouron developed a system with three analysers to measure phosphate, nitrate, and silicate together [110]. And Zhang et al. fabricated a self-supported electrode to detect ammonia based on electrodepositing platinum-polypyrrole on Ni foam [120]. New technologies also promote the improvement of old ones. For example, scientists improved the sensitivity measurement of nitrate concentration based on new dispersion turning point (DTP) theory [34]. Meanwhile, the emergence of new technologies also brings more detection methods. For example, phosphate was detected by interfacial barrier effects of p-n junction on electrochemistry [121]. (2) Some problems previously hard to study are solved. It was well known that not extensively researches had studied voltammetric sensors for nitrate detection in seawater, because the LOD data from sensors was usually above 1 μM, higher than the nitrate concentrations in seawater, particularly the concentration of nitrate at the ocean surface (at a nanomolar level) [122]. Nine years later, Legrand et al. developed an electrode by electrodepositing silver nanoparticles on a gold disc electrode to test nitrate in synthetic seawater [54]. The sensor showed a limit of quantification of 0.39 μM and a linear range of 0.39-50 μM. The peak current intensity remained at 95% of the initial value after regular detection of 25 μM nitrate for about 26 days. And (3) more and more emerging contaminants interference is still a technical challenge. For example, the challenges of nitrate biosensors based on reductases include oxygen interference [123], low electron transfer efficiency [124, 125], and high cost and low storage temperature, which promote relevant research and generate more literatures.

Although the quantity of relevant literatures has been increasing steadily, the research status within both analytes and detection methods is very varied. For analytes, we can see that more research focused on nitrate, phosphate, and ammonium. This result should attribute to seriously increasing marine environmental and ecological problems, such as frequent red tide, green tide, and other ecological disasters [9, 10, 126]. Hence, for a period of time, literatures related to nitrate, phosphate, ammonium, and phosphate will continue to increase with more accurate data and new detection methods.

For the detection methods, we found a similar trend as one previous review paper that spectrophotometry, colorimetry, fluorimetry, and electrochemistry are the most commonly used sensors for the detection of nutrient detection in seawater [127]. These traditional and classic detection methods have been attracting a lot of attention for years, followed by emerging chromatography, microfiber, microfluidics, optofluidics, and carbon nanotube (Table 2). Then, other electrochemical sensors have been used for water analysis. As we know, we widely used electrochemical sensors as one of the most promising analytical tools for the rapid detection of nitrate due to its high sensitivity, quick response, ease of operation and miniaturization, low sample and power consumption, low reagent consumption, and easy combination with automation devices [18, 128]. Nevertheless, some new methods also appeared with technical advantage. For example, a new method using a microfiber mode interferometric sensor to improve the sensitivity of nitrate concentration measurement in seawater based on dispersion turning point (DTP) theory is demonstrated [34]. Through interdisciplinary research within electronics, chemistry, materials science, etc., the application of microfluidic technology shows more advantages and gradually becomes a better way to further reduce energy consumption, reduce the amounts of reagents, and promote the miniaturization of sensors [129]. Therefore, more studies should focus on the development of new detection methods for clearer understanding of the nutrient in seawaters.

3.2. Sensitivity of Nutrient Sensors in Seawater

LODs in Table 1 show thousands of orders for different analytes and detection methods. Here, the average LOD of nutrient sensors included was μM. Among all the sensors checked, the most sensitive sensor was developed by Chen and Chumbimuni-Torres in which the indirect detection of nitrate through spectrophotometry and electrochemistry method was used [98, 103]. Above sensor could achieve an ultrasensitive LOD, as low as 0.1 nM [103]. By contrast, some other sensors are relatively “insensitive” with the LODs at the level of 2-10 μM, distributed from 2006 to 2016 [66, 68, 95, 104].

Figure 1(a) shows the average LOD of five nutrients at the level almost below1 μM with descending sequence: nitrate (1.0983) > silicate (0.5495) > phosphate (0.4823) > ammonium (0.1324) > nitrite (0.0568). Figure 1(b) only shows the average LOD of five detection methods with more than five literatures at the level almost below 1 μM (except for microfluidics at the level of 2 μM) with descending sequence: microfluidics (1.7617) > electrochemistry (1.2607) > colorimetry (0.4462) > spectrophotometry (0.2941) > fluorimetry (0.0558). For the rest four detection methods, chromatography, microfiber, optofluidics, and carbon nanotube have a LOD of 0.2200, 2.7419, 0.0451, and 0.01, respectively.

In general, both detection of nitrite and ammonium and fluorimetry method seem to have lower LODs (Figure 1). Nevertheless, we cannot just draw a simple conclusion that these related research develop best. This result could mainly attribute to the following: (1) some new literatures based on special detection technique usually increase the mean value with their own high LODs. For example, Yang et al. used a new technique to improve the sensitivity of sodium nitrate concentration measurement in seawater based on dispersion turning point [34]. Although the sensor shows advantages of easy to construct, low cost, high precision, and high sensitivity, which provides a new optical detection method for in situ detection of marine environment or low concentration substances sensing in other liquids, its LOD is 2.7419 μM. The same situation applies to Khongpet et al. who used a compact hydrodynamic sequential injection system for consecutive online determination of phosphate and ammonium, with a LOD 1.8947 μM [37]. On a long view, these technologies are constantly developing with lower LOD. And more literatures related to these technologies will finally lower the average LOD. Before we get too excited about that, we may have to face the situation that the emergence of other new technologies (in embryonic stage) may increase the mean value with their own high LODs, again. (2) Different detection methods have their own special value. For example, a variety of approaches have been used for the determination of phosphate in seawater, including colorimetric detection, fluorescent detection, and electrochemical detection. For all are reagent-based methods, phosphate cannot be detected directly. Therefore, autonomous systems tend to employ the colorimetric method rather than fluorescent or electrochemical methods [130]. Previous study has shown that when algal blooms erupt, the nutrient concentration in seawater is on the order of 10⁻⁹ mol/L [131]. So it is very necessary to utilize more appropriate methods to increase the accuracy of the sensor. In addition to accuracy, the nutrient concentration in the same sea area varies significantly at different times or in different sea areas and the ranges from 10⁻⁹ to 10⁻⁶ mol/L, with a difference of 5 orders of magnitude [131]. Hence, an ideal sensor should better have wide measurement range.

3.3. Comparison of Detection Methods

Figure 2 shows the result of analysis of one-way ANOVA for different detection methods (p < 0.05). In general, at the present stage, we could see that not all methods (NO DATA) can be used for detection within one nutrient in seawaters such as chromatography to nitrite, phosphate, ammonium, and silicate (Figure 2). Clearly, future studies should focus on the development of chromatography sensors applied in seawater.

Furthermore, except ammonium (p < 0.05), there were no significant differences among different detection methods applied in any nutrient sensor. This result indicated that, for nitrate, nitrite, phosphate, and silicate, their development level of detection methods is closer to each other. For ammonium, spectrophotometry has a significantly lower LOD than electrochemistry (p < 0.05), and fluorimetry also has a significantly lower LOD than electrochemistry (p < 0.05). This result is in accordance with the consensus that electrochemical sensors have been widely used as one of the most promising analytical tools for the rapid detection of nitrate in environmental matrices due to low sample and power consumption, high sensitivity, quick response, and ease of operation and miniaturization [18, 128].