Abstract
In this study, for the first time, to our knowledge, a biosensor was produced using cold atmospheric plasma (CAP) treatment of silver thin film surface with non-perpendicular incidence angle for identification of Escherichia coli in distilled water. Field emission scanning electron microscopy (FESEM) exhibits that before deposition, non-perpendicular CAP treatment of glass surface substrate leads to the production of pinhole-free silver thin film. The results of atomic force microscopy (AFM) and curve fitting show that non-perpendicular CAP treatment of this pinhole-free silver thin film indicates to the appearance of Ag NPs with smaller size and larger surface area compared to untreated silver film deposited on the untreated glass substrate. The silver-based pinhole-free SPR biosensor produced with non-perpendicular CAP treatment of both glass substrate and silver film shows E. coli detection in the distilled water in the range of 104 colony forming unit (CFU/ml) to 108 CFU/ml with better sensitivity compared to the untreated silver-based SPR biosensor.
HIGHLIGHTS
The glass substrate was treated using CAP for improving the silver film adhesion.
FESEM analysis was performed for observation of pinholes in thin film.
A silver-based SPR biosensor was fabricated using non-perpendicular CAP treatment for E. coli detection in water.
The effect of non-perpendicular CAP on the performance parameters of pinhole-free silver biosensor was studied.
Graphical Abstract
INTRODUCTION
Bacterial pathogens are important targets for detection and identification in medicine and food safety. Escherichia coli O157:H7 (pathogenic Gram-negative bacterium) is a common example of the leading cause of waterborne illnesses (Ahmed et al. 2014). This bacterium is non-spore forming and rod shaped, approximately 0.5 μm in diameter and between 1.0 and 3.0 μm in length (Rompré et al. 2002). So far, such different methods as culture-based detection or enzyme-linked immunosorbent assay (ELISA) have been developed for the detection of E. coli. Due to the high sensitivity, an optical detection method based on absorption, fluorescence, refraction, and reflection is the most widely used in pathogen monitoring systems (Nurliyana et al. 2018). In the last decades, the demand for development of such label-free optical biosensors as surface plasmon resonance (SPR) has increased for biological sensing (Tokel et al. 2015). The surface plasmon resonance (SPR) system is a label-free technique that does not require any tag to identify biomolecules such as nucleic acids, proteins, small molecules, and cells. Due to the capability for real-time monitoring of the biomolecular interactions with high sensitivity, the SPR biosensor has gained interests in the field of foodborne pathogen detection (Homola 2003). Since the SPR-based biosensor is very sensitive to changes in the refractive index of the analyte, this type of biosensor can be used to detect the existence of biomolecules (Sun 2014). Compared to gold film, silver-based SPR sensors exhibit higher sensitivity and a sharper reflectivity spectrum (Wang et al. 2017; Muhktar et al. 2018; Zainuddin et al. 2019; Kamkar et al. 2021). Indeed, using a highly sensitive and less expensive noble silver instead of gold, such drawbacks of gold-based SPR sensors as a wider SPR curve can be improved (Situ et al. 2010; Moznuzzaman et al. 2020). Additionally, it has been reported that silver metal gives an SPR curve with more plasmonic effect and narrowness than that of gold due to the high SPR ratio; this leads to a better signal-to-noise ratio (SNR) (Moznuzzaman et al. 2020). Hence, many researchers are developing silver-based SPR biosensors (Rahman et al. 2017a, 2017b, 2019). The drawback of silver such as oxidation (Verma et al. 2015) can also be reduced using molybdenum disulfide (MoS2) or graphene (Homola 2003).
Generally, in SPR biosensors with gold thin film, such probe molecules as enzymes (Peltomaa et al. 2018) are firstly immobilized for binding the target biomolecule to the sensor surface. However, Ag NPs can anchor to the bacterial cell wall (Qing et al. 2018). After adhesion to the cell, Ag NPs can also penetrate the bacterial membrane, interact with cellular structures and biomolecules such as proteins (Wigginton et al. 2010) and bind to the bacterial proteins, referred to as protein adsorption (Polini & Yang 2017). The adsorption of such biomolecules as bacterial proteins with Ag NPs changes the reflective index at the sensor surface; leading to the detection via SPR biosensors based on SPR measurements.
Surface hydrophilicity or wettability is one of the influential parameters to control cell behavior by protein adsorption with nanoparticles (Polini & Yang 2017). Water contact angle measurement (WCA) is the most common method used to find the surface wettability of a solid by a liquid drop (Chen et al. 2018). So, in this study, the water contact angle measurement was utilized to control silver thin film surface qualities such as roughness or heterogeneity for enhancing bacterial protein adsorption.
It has been reported that the perpendicular CAP treatment can improve Ag NPs interaction with gram-negative bacteria cells by decreasing nanoparticle (NP) size (Manaloto et al. 2020) and enhancing the surface hydrophilicity (Zendehnam et al. 2018). Furthermore, it has been proved that the non-perpendicular CAP treatment of silver thin film results in the significant variations in the Ag NPs size (Hosseinpour et al. 2019). In addition, the CAP treatment of the glass substrate improves the Ag NPs deposition rate, increases the thin film adhesion as well as the density and compactness of the deposited Ag NPs, leading to the fabrication of pinhole- free thin films (Lisco et al. 2017), and resulting in bacterial detection based on protein adsorption (Wigginton et al. 2010). CAP treatment is an environmentally benign technology, considered as a surface modification technique for such materials as polymers (Reche et al. 2016), or inorganic materials (e.g. metals) (Kawase et al. 2014; Hosseinpour et al. 2019). Hence, in this study, the positive effect of non-perpendicular CAP treatment on the surface of silver thin film and its glass substrate was used for the biosensor production and the SPR signals were measured using the intensity-wavelength interrogation methodology with the help of a spectrophotometer.
To investigate the influence of the non-perpendicular CAP treatment on the device performance, here, the sensitivity of non-perpendicular CAP-treated silver thin film deposited on the non-perpendicular CAP-treated glass substrate was compared to the sensitivity of untreated silver thin film deposited on the untreated glass for identification of E. coli bacteria in the distilled water.
One critical challenge for SPR measurements is to correctly build an SPR sensing platform. There are four types of methodology to measure the SPR signal. The intensity modulation is the first one, in which the principle is to detect the intensity variation of the reflectivity in particular angle or wavelength (Prabowo et al. 2018). In this methodology, as a spectrophotometer is utilized, an applicable sensor chip holder is necessary to suspend the chip correctly in the standard spectrophotometer flow cells containing the sample solutions with various concentrations for recording the reflectivity spectra. Wang et al. utilized a complicated flow cell to measure SPR signals (Wang et al. 2017). However, in this work, a simple and low-cost chip holder was designed using a computer-aided design (CAD) model and fabricated by the use of 3D printing technology (Shahrubudin et al. 2019) to facilitate SPR measurement with standard spectrophotometer flow cells. This sensor holder has the advantage of being reusable for a number of SPR measurements since it can be properly cleaned by the use of such disinfectant liquids as bleach.
MATERIALS AND METHODS
APPJ-set up
To produce a sensor chip using CAP treatment, a schematic representation of the experimental set-up used is shown in Figure 1. APPJ consisted of dielectric, powered, and grounded electrodes and a 6 kHz high voltage pulsed DC power supply. A copper rod (length = 30 mm, diameter = 1 mm) and a thin cylindrical copper tube (length = 15 mm) used as powered and grounded electrodes, respectively. A Pyrex tube (length = 150 mm, OD = 6 mm, ID = 4 mm) was employed as the dielectric barrier (the jet nozzle). The powered electrode was inserted in the Pyrex tube from one end, while the other tube end was surrounded by the grounded electrode. The distance between the nozzle tip and the lower edge of the grounded electrode was 5 mm. Moreover, argon gas with a purity of 99.999% was used as feeding gas with a flow rate of 2slm. CAP applications are mostly based on the generation capability for sufficient amounts of various reactive species (Rezaeinezhad et al. 2019). So, optical emission spectroscopy (OES) was used to analyze the existence and intensity of any species. For this purpose, an Ocean Optic HR 2000 spectrometer was employed with the spectral range of 200–1,100 nm and optical resolution of 0.5 nm. The plasma optical emissions recorded at a distance of 1 cm from the plasma stream. According to the atomic spectra database lines, the recorded spectrum was analyzed and different species were determined.
(a) Image of the APPJ, 3D schematic of the APPJ in Auto CAD with (b) X-ray visual style and (c) shaded visual style.
(a) Image of the APPJ, 3D schematic of the APPJ in Auto CAD with (b) X-ray visual style and (c) shaded visual style.
Sputtering silver film on the CAP- treated and untreated glass substrates
Silver thin film was prepared by DC magnetron sputtering (Hind High Vacuum, H.H.V, 12″MSPT) of silver onto untreated and CAP-treated square glass substrates (1 × 1 cm2). It has been reported that based on such SPR signal analyses as Q-factor and full width half maximum (FWHM), a thickness of 45 nm or 55 nm silver layer can generate an excellent amount of surface plasmon polaritons (SPP) due to its low FWHM and high Q-factor values (Muhktar et al. 2018). Hence, the thicknesses of thin films were 45 nm with a deposition rate of 1.5 nm°/S. The working condition of the magnetron sputter was 195 W with a voltage of 325 V, Ag target (99.9% purity), substrate to target distance 12 cm with argon as the working gas (purity 99.99%) and pressure of 3 × 10−2 mbar (base pressure 10−5 mbar). The coating rate and silver film thickness were measured using a vibrating quartz crystal thickness monitor.
CAP treatment of the glass substrate
It has been reported that deposition techniques such as sputtering, electrodeposition, and spin coating require surface treatment of the substrate with atmospheric pressure plasma prior to the deposition of the thin film (Lisco et al. 2017). This is due to the fact that atmospheric-pressure plasma treatment of the substrate improves the Ag nanoparticles' (NPs) deposition rate and increases the thin film adhesion as well as the density and compactness of the deposited Ag NPs, leading to the fabrication of pinhole- free thin films (Lisco et al. 2017). The presence of pinholes in the silver thin film surface indicates the sparse distribution of bacteria along the sensor surface. Therefore, in some parts of the sensor no bacterium is present; resulting in the weaker excitation of the SPs along the metal-dielectric interface. Hence, here, CAP treatment of glass substrate with angle of 60°, treatment time 2 min, applied voltage 10 KV, nozzle-glass distance 1 cm and gas flow rate 2slm was used before deposition.
CAP treatment of the silver thin film deposited on the CAP-treated glass substrate
Based on the discussions in the Introduction section, different CAP treatments including 90°(1 min) + 60°(1 min), 60°(1 min) + 90°(1 min), and 60°(2 min) were used to decrease the Ag NPs size ; increase the Ag NPs surface area for improvement in binding of silver nanoparticles to bacterial proteins. Based on the WCA measurements, AFM and curve fitting results, among the examined conditions, the most effective CAP treatment condition for surface hydrophilicity and protein adsorption improvement of Ag NPs were obtained to produce the silver-based SPR biosensor. However, the rest of the CAP treatment parameters were kept constant (applied voltage 8 KV, treatment time 2 min, applied frequency 6 KHz, nozzle-silver thin film distance 1 cm and gas flow rate 2slm).
Characterization of the glass substrate before and after CAP treatment
Measuring the contact angle of a water droplet on the solid surface is used for surface quality control in coating technology (Chen et al. 2018). Before and after cold atmospheric-pressure plasma treatment, the changes in the wettability of glass substrate surface were evaluated using water contact angle measurements at 25 °C by placing a 5 μl droplet of distilled water at the glass surface and capturing an image of drop with a camera a few seconds after deposition of the drop. To analyze the drop image and calculate the contact angle value, an image analysis program, Image J (Drop analysis- LB-ADSA) was used (Williams et al. 2010).
The surface morphology of the glass substrate was also investigated before and after CAP treatment using AFM by a Nanosurf Mobile S at scanning area 9 μm × 9 μm.
Characterization of the untreated silver thin film deposited on the untreated glass substrate and CAP-treated silver thin film deposited on the CAP-treated glass
Since surface hydrophilicity can be used to control cell behavior by bacterial protein adsorption for sensing application, before and after CAP treatment of silver thin film, the WCA values were measured. For this purpose, at 25 °C, 5 μl drops of distilled water were placed at the silver thin film surfaces and images captured a few seconds after deposition of the drop.



Field emission scanning electron microscopy (FESEM) of the silver thin film deposited on CAP-treated glass substrate was also carried out (JEOL, JSM 7800F) at an accelerating voltage of 25 KV to show the improvements of thin film adhesion after CAP treatment of the glass substrate.
Sensing setup
Design of sensor holder using 3D printing technology
Based on the discussion in Section 1, we first designed the 3D structure of the sensor holder using Auto CAD v2015 software. The binder jetting process of 3D printing technology was utilized to fabricate the CAD model with ABS material (Fig. S1 in the supplementary material). The step-by step principle of plotting and designing a 3D structure of the chip holder using a CAD model (v2015) is shown in Fig. S2.
SPR measurements
A schematic diagram of the experimental setup used for the present study is shown in Figure 2. To prepare a bacterial sample, an E. coli solution was diluted to lower concentrations (,
,
,
,
CFU/ml) with distilled water. These bacterial sample solutions with varying concentrations were respectively filled into the flow cells (Figure 2) using a 2cc (cubic centimeter) insulin syringe. The sensor chips were correctly embedded and fixed in the produced ABS sensor holders (Figs. S1 (b), (c)) by 3D printer and suspended in the spectrophotometer flow cells containing various concentrations of E. coli, respectively. The flow cells were placed in the spectrophotometer instrument with the help of a 3D translational stage (see Fig. S1 (d)). Finally, the SPR measurements were recorded using a spectrophotometer (Perkin Elmer Lambda 25 with an optical resolution of 0.5 nm) based on the intensity-wavelength interrogation methodology (Prabowo et al. 2018) with the help of experimental setup in Figure 2.
Performance parameters of the SPR sensor


The average sensitivity of the sensors is also calculated by the slope of a linear fit to the calibration data when the (nm) is plotted as a function of log E.coli concentration (cfu/ml) (Dormeny et al. 2020).








Stability tests of the untreated and CAP-treated silver-based SPR sensor
Hydrophobic recovery



Reflectivity spectra
The reflectivity spectra of the untreated and CAP-treated silver thin films were recorded in air environment over time (24 h, 48, 72 and 96 h) and the shifts in the resonance wavelength were used as a measure of the optical stability of the produced sensors. For this purpose, prior to the WCA measurements, the reflectivity spectrum for each sample was recorded over time (24 h, 48, 72 and 96 h) using a spectrophotometer (Perkin Elmer Lambda 25 with an optical resolution of 0.5 nm) based on the intensity-wavelength interrogation methodology (Wang et al. 2017; Prabowo et al. 2018).
RESULTS AND DISCUSSION
OES measurement
The emission spectrum of the employed argon plasma jet in the air medium is illustrated in Figure 3(a) for a non-perpendicular and in Figure 3(b) for a perpendicular incidence angle of the jet, respectively. The multiple transition lines were observed in the optical emission spectrum of the plasma jet afterglow operating in ambient air. Accordingly, based on atomic spectra database lines, an intense atomic oxygen line at 777 nm, an intense OH rotational band at 308.5 nm and strong atomic argon emission lines in the range 690–851 nm can be observed.
OES spectrum of the (a) non-perpendicular (jet angle: 60°) and (b) perpendicular (jet angle: 90°) argon plasma jet.
OES spectrum of the (a) non-perpendicular (jet angle: 60°) and (b) perpendicular (jet angle: 90°) argon plasma jet.
However, some low intensity emissions are also obtained: emission due to the presence of NO radicals at 282.83 nm, and emissions attributed to atomic oxygen at 927 nm. Moreover, as shown in Figure 3, lower intensity of OH rotational band at 308.5 nm was observed for a non-perpendicular incidence angle of jet compared to the perpendicular incidence angle.
CAP-treated glass substrate characterization
As presented in Fig. S3, the remarkable increment (about 6 nm) in the roughness of the glass surface substrate was observed after CAP treatment with plasma exposure of 60° under the mentioned operating conditions in the ‘CAP treatment of the glass substrate’ section. Based on the Wenzel model (Nosonovsky & Bhushan 2009) with an increase in roughness (Figure 4(a)), a hydrophilic surface (WCA of glass substrate before CAP treatment < 90°) becomes more hydrophilic (Figure 4(b): WCA of glass substrate after non-perpendicular CAP treatment
). This CAP treatment of glass substrate leads to the enhancement in the surface hydrophilicity of glass substrate about 12.5° (See Figure 4), resulting in the improvement in the adhesion of thin film to the glass surface with more uniformity of silver nanoparticles; indicating to the production of pinhole-free silver thin film, as shown in FESEM images (Figure 5).
WCA of the glass substrate (a) before and (b) after non-perpendicular CAP treatment with jet angle 60° at treatment conditions of applied voltage 10 KV, nozzle-glass distance 1 cm, gas flow rate 2slm, treatment time 2 min obtained using Image J software (Drop analysis- LB-ADSA).
WCA of the glass substrate (a) before and (b) after non-perpendicular CAP treatment with jet angle 60° at treatment conditions of applied voltage 10 KV, nozzle-glass distance 1 cm, gas flow rate 2slm, treatment time 2 min obtained using Image J software (Drop analysis- LB-ADSA).
FESEM images of (a) untreated silver thin film deposited on the untreated glass (cleaned only with acetone in ultrasonic bath-2 min) and (b) untreated silver thin film deposited on the non-perpendicular CAP-treated glass substrate using jet angle 60°, gas flow rate 2slm, applied voltage 10 KV, at a magnification of 80,000 x. The orange arrows indicate holes in the untreated silver thin film deposited on the untreated glass. The full colour version of this figure is available in the online version of this paper, at http://dx.doi.org/10.2166/ws.2021.282.
FESEM images of (a) untreated silver thin film deposited on the untreated glass (cleaned only with acetone in ultrasonic bath-2 min) and (b) untreated silver thin film deposited on the non-perpendicular CAP-treated glass substrate using jet angle 60°, gas flow rate 2slm, applied voltage 10 KV, at a magnification of 80,000 x. The orange arrows indicate holes in the untreated silver thin film deposited on the untreated glass. The full colour version of this figure is available in the online version of this paper, at http://dx.doi.org/10.2166/ws.2021.282.
Characterizations of untreated and CAP-treated silver thin films
Wettability
Based on the discussion in the ‘Introduction’ section, surface hydrophilicity improvement can play important role in protein adsorption; indicating the bacterial detection (Wigginton et al. 2010) and CAP treatment is a promising technique for this purpose. According to WCA measurements, as presented in Figure 6(a) and 6(b), a remarkable reduction in the WCA value (about 54.63°) was observed after CAP treatment with 90°(1 min) + 60°(1 min). However, insignificant reductions were obtained in the WCA values for the other examined CAP treatments, as shown in Figure 6(c) and 6(d), which can be explained by the results of AFM and curve fitting of Ag NPs using the XY Extract Graph Digitizer and Origin Pro software. As can be seen in Fig. S6 (d), the surface roughness of Ag film treated with 90°(1 min) + 60°(1 min) exhibits a remarkable increase (about 7 nm) when compared to Ag film treated with 60°(1 min) + 90°(1 min). Additionally, compared with the Ag film treated with 60°(1 min) + 90°(1 min), CAP treatment with 90°(1 min) + 60°(1 min) can remarkably increase the surface area of Ag NP (Fig. S6(c)) and decrease the FWHM of Ag NP (Fig. S6(b)), which finally leads to the larger enhancement in the hydrophicility of the Ag film treated with 90°(1 min) + 60°(1 min). The results of OES measurements in Figure 3 indicate that the plasma exposure at 90° shows a slightly higher OH intensity compared to plasma emission at 60°; however, the intensity of OH species is still minor and cannot be the major factor for influencing surface roughness and hydrophicility of Ag film.
WCA of (a) untreated silver thin film and non-perpendicular CAP- treated silver thin film for (b) 90°(1 min) + 60°(1 min), (c) 60°(1 min) + 90°(1 min), and (d) 60°(2 min) plasma exposures obtained using Image J software (Drop analysis- LB-ADSA).
WCA of (a) untreated silver thin film and non-perpendicular CAP- treated silver thin film for (b) 90°(1 min) + 60°(1 min), (c) 60°(1 min) + 90°(1 min), and (d) 60°(2 min) plasma exposures obtained using Image J software (Drop analysis- LB-ADSA).
Morphological studies with curve fitting for wettability improvement
It has been reported that smaller Ag NPs has a large surface area in contact with the bacterial cellular structures, leading to the surface hydrophilicity or protein adsorption improvement (Wigginton et al. 2010; Polini & Yang 2017).
The Origin Pro software using curve fitting technique has the advantages of calculating the surface area and full width at half maximum (FWHM) for Ag NP size evaluation with the help of 1D AFM profiles shown in Fig. S5(a) (before CAP treatment) and after CAP treatments (Fig. S5(b) and Fig. S5(c)), respectively . Using this technique, as presented in Fig. S6 (a), Fig. S6 (b), and Fig. S6(c), after CAP treatment with 90°(1 min) + 60°(1 min), the surface area of Ag NPs remarkably increases due to the increment in the height of Ag NPs. However, as is shown in Fig. S6 (b), the noticeable decrement in the FWHM of Ag NP for the silver film treated with 90°(1 min) + 60°(1 min) was observed; indicating to the appearance of NPs with smaller size than both untreated silver thin film and CAP-treated silver thin film with 60°(1 min) + 90°(1 min).
In addition, after CAP treatment of 90°(1 min) + 60°(1 min), more uniform distribution of Ag NPs with smaller size on the surface was appeared (see Fig. S4).
Comparison between the performances of the untreated and CAP-treated silver-based SPR biosensors
It would be interesting to compare the sensitivities of the CAP-treated pinhole-free silver thin film and the untreated silver thin film deposited respectively on the CAP-treated and untreated glass substrates. Hence, the spectra of the reflected light before immersing the untreated silver thin film SPR biosensor and CAP-treated SPR biosensor inside the E. coli solutions respectively are shown in Figures 7(a) and 8(a). According to Figures 7(a) and 8(a), the strength of the SPR signal became weaker at the resonance wavelength for the untreated SPR biosensor due to the presence of pinholes in the silver film surface deposited on the untreated glass substrate, as presented in the FESEM image (see Figure 5(a)). As discussed previously, based on the intensity-wavelength interrogation methodology for SPR measurements the local refractive index slightly changes, giving rise to a change in the SPR resonance wavelength due to the protein adsorption with Ag NPs (Nguyen et al. 2015). Thus, when the concentration of the bacterial sample around the sensor surface increases due to the protein adsorption of bacterial cells with Ag NPs, the SPR resonance wavelength exhibits a red shift, indicating the detection (Nguyen et al. 2015), as can be seen in Figures 7(b) and 8(b), respectively, for untreated and CAP-treated silver thin films with 90°(1 min) + 60°(1 min). In addition, as shown in Figures 7(b) and 8(b), the wavelength shifts to higher value with increasing the concentration of E. coli. This indicates the fact that the biological binding reactions between the Ag NPs and E. coli bacteria increase as the concentration of E. coli increases; which finally leads to the enhancement in the concentration of bacteria on the sensor surface (Nguyen et al. 2015). The variations of resonance wavelength with changes in E. coli concentrations and the corresponding calibration curves are illustrated in Figure 7(c) for untreated silver thin film and in Figure 8(c) for CAP-treated silver thin film, respectively. A good linearity in the calibration curve is illustrated in Figures 7(c) and 8(c); indicating the applicability of the SPR sensors for detecting the E. coli bacterium. The average sensitivity of the CAP-treated SPR sensor is 4.1 , which is calculated using the slope of a linear fit to the calibration curve, as shown in Figure 8(c). However, the untreated silver thin film shows the lower average sensitivity, and it is equal to 3.3
, as illustrated in Figure 7(c).
(a) Spectrum of the reflected light before immersing the untreated SPR biosensor inside the E. coli solution; (b) SPR spectra of the untreated SPR biosensor for varying log E. coli concentrations ; (c) variation of the resonance wavelength with log E. coli concentration
for the untreated SPR biosensor, The linear line fitted our data with R2 = 0.9829, the average sensitivity of 3.3
is calculated by the slope of the calibration curve.
(a) Spectrum of the reflected light before immersing the untreated SPR biosensor inside the E. coli solution; (b) SPR spectra of the untreated SPR biosensor for varying log E. coli concentrations ; (c) variation of the resonance wavelength with log E. coli concentration
for the untreated SPR biosensor, The linear line fitted our data with R2 = 0.9829, the average sensitivity of 3.3
is calculated by the slope of the calibration curve.
(a) Spectrum of the reflected light before immersing the CAP-treated silver-based SPR biosensor inside the E. coli solution; (b) SPR spectra of the CAP-treated silver-based SPR biosensor (pinhole-free) for varying log E. coli concentrations ; (c) variation of the resonance wavelength with log E. coli concentration
for the CAP-treated pinhole-free silver-based SPR biosensor. The linear line fitted our data with R2 = 0.9819, the average sensitivity of 4.1
is calculated by the slope of the calibration curve.
(a) Spectrum of the reflected light before immersing the CAP-treated silver-based SPR biosensor inside the E. coli solution; (b) SPR spectra of the CAP-treated silver-based SPR biosensor (pinhole-free) for varying log E. coli concentrations ; (c) variation of the resonance wavelength with log E. coli concentration
for the CAP-treated pinhole-free silver-based SPR biosensor. The linear line fitted our data with R2 = 0.9819, the average sensitivity of 4.1
is calculated by the slope of the calibration curve.
The performance parameters for the SPR biosensors are listed in Tables 1 and 2, respectively. Results of the performance parameters in Table 1 indicate that as the concentration of E. coli is increased from 104 CFU/ml to 108 CFU/ml in the untreated silver based SPR sensor, the sensitivity increases from 0.5 to 1.88
, but the FWHM does not change significantly. This leads to the increasing of the FOM. Similarly, the results in Table 2 exhibit that increasing the concentration of E. coli from 104 CFU/ml up to 108 CFU/ml in the CAP-treated silver-based SPR sensor can increase sensitivity from 1.5
to 2.75
while the FWHM does not change significantly; thus, this leads to the FOM enhancement. Additionally, based on the Equation (5), the CAP-treated silver thin film sensor exhibits a slightly higher average resolution
when compared with the untreated silver thin film sensor
. Compared with the CAP-treated silver thin film (pinhole-free SPR sensor: Table 2), the untreated silver thin film (SPR sensor with pinholes) exhibits smaller resonance wavelengths, slightly lower FWHM with also decreasing sensitivity; which finally leads to decreasing FOM, as can be seen in Table 1 for the E.coli concentration from104 CFU/ml up to 108 CFU/ml.
Detailed results of the performance parameters for the untreated silver-based SPR sensor
E. coli concentration (CFU/ml) . | Resonance wavelength (nm) . | Reflectance (a.u.) . | Sensitivity ![]() | FWHM (nm) . | FOM ![]() |
---|---|---|---|---|---|
104 | 327 | 0.138 | 0.5 | 7.667 | 0.065 |
105 | 330 | 0.158 | 1 | 7.619 | 0.131 |
106 | 335 | 0.198 | 1.67 | 7.781 | 0.215 |
107 | 337 | 0.228 | 1.71 | 7.885 | 0.217 |
108 | 340 | 0.248 | 1.88 | 7.948 | 0.237 |
E. coli concentration (CFU/ml) . | Resonance wavelength (nm) . | Reflectance (a.u.) . | Sensitivity ![]() | FWHM (nm) . | FOM ![]() |
---|---|---|---|---|---|
104 | 327 | 0.138 | 0.5 | 7.667 | 0.065 |
105 | 330 | 0.158 | 1 | 7.619 | 0.131 |
106 | 335 | 0.198 | 1.67 | 7.781 | 0.215 |
107 | 337 | 0.228 | 1.71 | 7.885 | 0.217 |
108 | 340 | 0.248 | 1.88 | 7.948 | 0.237 |
Detailed results of the performance parameters for the CAP-treated silver-based SPR sensor
E. coli concentration (CFU/ml) . | Resonance wavelength (nm) . | Reflectance (a.u.) . | Sensitivity ![]() | FWHM (nm) . | FOM ![]() |
---|---|---|---|---|---|
104 | 330 | 0.058 | 1.5 | 8.171 | 0.184 |
105 | 333 | 0.088 | 1.8 | 8.328 | 0.216 |
106 | 336 | 0.138 | 2 | 8.468 | 0.236 |
107 | 342 | 0.158 | 2.57 | 8.417 | 0.305 |
108 | 346 | 0.182 | 2.75 | 8.511 | 0.323 |
E. coli concentration (CFU/ml) . | Resonance wavelength (nm) . | Reflectance (a.u.) . | Sensitivity ![]() | FWHM (nm) . | FOM ![]() |
---|---|---|---|---|---|
104 | 330 | 0.058 | 1.5 | 8.171 | 0.184 |
105 | 333 | 0.088 | 1.8 | 8.328 | 0.216 |
106 | 336 | 0.138 | 2 | 8.468 | 0.236 |
107 | 342 | 0.158 | 2.57 | 8.417 | 0.305 |
108 | 346 | 0.182 | 2.75 | 8.511 | 0.323 |
Stability of the untreated and CAP-treated silver-based SPR sensor
Figure 9 shows the evolution of the WCA as a function of storage time for the CAP-treated silver thin films, respectively. Figure 10 illustrates the reduction in the WCA values of the CAP-treated silver thin films over the 4 days (96 h) following the treatment (loss in treatment efficiency (%)) obtained using Equation (7) (Deynse et al. 2015). As can be seen in Figures 9 and 10, the CAP-treated silver thin film still shows hydrophilicity (62.56° < 90°) with loss in treatment efficiency of only 35.46% in 96 h from CAP treatment.
WCA values of the CAP-treated silver thin films over time in ambient environment (a) 24 h, (b) 48 h, (c) 72 h, and (d) 96 h.
WCA values of the CAP-treated silver thin films over time in ambient environment (a) 24 h, (b) 48 h, (c) 72 h, and (d) 96 h.
The loss in treatment efficiency (%) values of the CAP-treated silver thin film as a fuunction of storage time in ambient environment.
The loss in treatment efficiency (%) values of the CAP-treated silver thin film as a fuunction of storage time in ambient environment.
Additionally, Figure 11(b) indicates that the reflectivity spectra are nearly the same over time (after 24 h, 48, 72 and 96 h of aging) for the CAP-treated silver thin film. This is probably due to the reduction in the surface activation of the CAP-treated silver thin film over the 96 h following the treatment (Figure 12). The chemical composition of the silver thin film surfaces were assessed by Fourier-transform infrared spectroscopy (FTIR) (400–4,000 cm−1) (FTIR spectrometer, Bruker; Alpha). Indeed, as can be seen in Figure 12, the intensity of all of the observed absorption bands in the FTIR spectra of the CAP-treated silver thin film was decreased over time (after 24, 48, 72 and 96 h of aging); leading to the weaker incorporation of such polar groups as N-H and O-H stretching vibration (alcohol and phenol) at 3,286 cm−1 as well as N = O symmetry stretching (nitro compound) at 1,378 cm−1 on the silver surface. As presented in Figure 11(a), a red shift can be observed in the reflectivity spectrum of the untreated silver thin film after 96 h of aging. So, one can conclude that the CAP-treated silver thin film is more stable in ambient environment than the untreated silver thin film.
Reflectivity spectra of (a) untreated silver thin film and (b) CAP-treated silver thin film over time (after 24 h, 48, 72 and 96 h of aging) in ambient environment.
Reflectivity spectra of (a) untreated silver thin film and (b) CAP-treated silver thin film over time (after 24 h, 48, 72 and 96 h of aging) in ambient environment.
FTIR spectra of the CAP-treated silver thin films immediately after treatment (0 h) and over time (after 24, 48, 72 and 96 h of aging) in ambient environment.
FTIR spectra of the CAP-treated silver thin films immediately after treatment (0 h) and over time (after 24, 48, 72 and 96 h of aging) in ambient environment.
Consequently, the results show that the applicability of the CAP-treated silver thin film (pinhole-free SPR biosensor) for identification of the E. coli is better than that of the untreated silver film. Here, two points must be carefully noticed and discussed in the measured experimental data. According to the FESEM images and WCA measurements, the first one is related to the noticeable effect of CAP treatment of the glass substrate on the Ag NPs deposition rate, the thin film adhesion as well as the density and compactness of the deposited Ag NPs; leading to the fabrication of pinhole-free thin films (Lisco et al. 2017); resulting in better bacterial detection based on protein adsorption compared to the untreated silver film containing pinholes (Wigginton et al. 2010). The second point is attributed to the remarkable influence of CAP treatment of this pinhole-free silver thin film on not only Ag NP size but also the distribution of Ag NPs in the CAP-treated glass surface. The AFM and curve fitting results show that the appearance of more uniform distribution of Ag NPs with smaller size (see Fig. S(5)b: left side) and larger surface area was observed on the silver thin film surface after CAP treatment (pinhole-free biosensor), leading to the sensitivity and FOM enhancement.
CONCLUSIONS
In this study, for the first time, to our knowledge, a pinhole-free silver-based SPR biosensor was produced using CAP treatment of silver thin film surface for identification of Escherichia coli in the distilled water. Also, for the first time, a simple and low-cost sensor holder is fabricated using a 3D printer for correctly suspending the produced silver thin film chip in the standard spectrophotometer flow cell containing bacteria solutions. FESEM results show that CAP treatment of the glass substrate with jet angle of 60° (2 min) improves the Ag nanoparticles (NPs) deposition rate and increases the thin film adhesion, leading to the fabrication of pinhole- free thin films. The results of AFM and curve fitting show that CAP treatment of this pinhole-free silver thin film with 90°(1 min) + 60°(1 min) indicates the appearance of Ag NPs with smaller size and larger surface area compared to untreated silver film deposited on the untreated glass substrate. The pinhole-free silver-based SPR biosensor produced with CAP treatment of both glass substrate and silver film shows better sensitivity, FOM and stability compared to the untreated silver-based SPR biosensor when the E. coli concentrations are increased from 104CFU/ml up to 108 CFU/ml. This improved performance is attributed to not only the omission of pinholes but also the appearance of more uniform distribution of Ag NPs with smaller size and larger surface area due to the CAP treatment.
AUTHOR CONTRIBUTIONS
Conceptualization: [Maryam Hosseinpour]; Methodology: [Maryam Hosseinpour]; Formal analysis: [Maryam Hosseinpour]; Investigation: [Maryam Hosseinpour]; Resources: [Hamidreza Ghomi Marzdashti, Maryam Hosseinpour]; Visualization: [Maryam Hosseinpour]; Supervision: [Akbar Zendehnam, Seyedeh Mehri Hamidi Sangdehi]; Writing – Original Draft :[Maryam Hosseinpour]; Writing – Review & Editing: [Seyedeh Mehri Hamidi Sangdehi, Maryam Hosseinpour]. All authors read and approved the final manuscript.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.