sensors Article
Figure of Merit Enhancement of a Surface Plasmon Resonance Sensor Using a Low-Refractive-Index Porous Silica Film Qing-Qing Meng 1 , Xin Zhao 1 , Cheng-You Lin 1, *, Shu-Jing Chen 2 , Ying-Chun Ding 1 and Zhao-Yang Chen 1, * 1
2
*
College of Science, Beijing University of Chemical Technology, Beijing 100029, China;
[email protected] (Q.-Q.M.);
[email protected] (X.Z.);
[email protected] (Y.-C.D.) School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China;
[email protected] Correspondence:
[email protected] (C.-Y.L.);
[email protected] (Z.-Y.C.); Tel.: +86-186-1008-1530 (C.-Y.L.); +86-150-1144-2703 (Z.-Y.C.)
Received: 7 July 2017; Accepted: 8 August 2017; Published: 10 August 2017
Abstract: In this paper; the surface plasmon resonance (SPR) sensor with a porous silica film was studied. The effect of the thickness and porosity of the porous silica film on the performance of the sensor was analyzed. The results indicated that the figure of merit (FOM) of an SPR sensor can be enhanced by using a porous silica film with a low-refractive-index. Particularly; the FOM of an SPR sensor with 40 nm thick 90% porosity porous silica film; whose refractive index is 1.04 was improved by 311% when compared with that of a traditional SPR sensor. Furthermore; it was found that the decrease in the refractive index or the increase in the thickness of the low-refractive-index porous silica film can enlarge the FOM enhancement. It is believed that the proposed SPR sensor with a low-refractive-index porous silica film will be helpful for high-performance SPR sensors development. Keywords: surface plasmon resonance sensor; figure of merit; porous silica; low-refractive-index
1. Introduction The surface plasmon resonance (SPR) sensor, as an important optical sensor, has been used in chemical and biochemical sensing, gases sensing, medical diagnostics, and food safety detection for years [1–3]. The rapid development of SPR sensor technology is owed to its distinctive features, such as label-free quantification, real-time analysis, and high sensitivity [4–6]. Among the various types of SPR sensors, one of the widely used geometries is the Kretschmann configuration which is generally prepared by evaporating a thin metal film on top of a glass prism. In this configuration, p-polarized light illuminates on the metal surface from the prism side to excite the surface plasmon wave (SPW) [7]. When the incident angle of the light source fulfills the SPR requirements, the propagation constant of incident light along the interface will match that of the SPW [8], causing the collective oscillation of electrons on the surface of the metal [9,10]. In this case, the energy of incident light transfers onto the surface plasmons, which leads to a significant decrease in the reflectance, and forms a narrow dip in the reflectance spectrum [11]. Reflectance dip shifts, caused by the change of the analyte refractive index, can be used for sensing. For instance, in the angular interrogation mode, a change in the analyte refractive index causes a shift in the resonance angle (at which the minimum reflectance is realized) for a given wavelength [12]. The figure of merit (FOM), defined as the ratio between the sensitivity (S) and the full width at half maximum (FWHM) of the reflectance dip, is a comprehensive parameter to evaluate the performance
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of an SPR sensor [5]. According to the definition of FOM, improving the sensitivity is an immediate way to achieve FOM enhancement. In recent years, many investigations have been carried out for sensitivity improvement. For example, Lahav et al. [13] facilitated sensitivity enhancement by using a guided-wave SPR structure, which was prepared by evaporating a 10 nm Si layer on top of the metal. Shukla et al. [14] and Bao et al. [15] both demonstrated that the sensitivity of an SPR sensor could be increased by adding a ZnO thin film. Besides, Benkabou et al. [16] achieved sensitivity enhancement Sensors 2017, 17, 1846 2 of 11 by employing the dielectric multilayer structure in the SPR sensor. performance an SPRdemonstrated sensor [5]. According to the definition of FOM, sensitivity layer is Although it hasofbeen that the addition of improving a thin the dielectric with a an immediate way to achieve FOM enhancement. In recent years, many investigations have been high-refractive-index on top of the metal layer can improve the sensitivity of an SPR sensor [11], the carried out for sensitivity improvement. For example, Lahav et al. [13] facilitated sensitivity FOM of theenhancement SPR sensor cannot be enhanced in thiswhich way.was The reason is as follows: by using a guided-wave SPR structure, prepared by evaporating a 10 nmthis Si sensitivity layer on top of induces the metal. Shukla et al. [14] and Bao et al. [15]of both demonstrated that dip, the sensitivity improvement method a significant broadening the reflectance which causes the an SPR sensor could be increasedleading by addingtoa ZnO thin film. or Besides, et al.in [16] achieved decreasing ofofthe FWHM, consequently no change, evenBenkabou a decrease FOM [17]. Compared sensitivity enhancement by employing the dielectric multilayer structure in the SPR sensor. with high-refractive-index dielectric thin films, the low-refractive-index dielectric thin films Although it has been demonstrated that the addition of a thin dielectric layer with a seem to be seldom usedhigh-refractive-index for the performance animprove SPR sensor. In this paper, we studied on topimprovement of the metal layerof can the sensitivity of an SPR sensor [11], the the effect of FOM of the SPR sensor cannot enhanced in this way. The reason is as follows: this sensitivity a low-refractive-index dielectric thinbefilm on the performance of an SPR sensor, which has not been improvement method induces a significant broadening of the reflectance dip, which causes the reported in literature yet. decreasing of the FWHM, consequently leading to no change, or even a decrease in FOM [17]. As an example a study, porous silica (a kind low-refractive-index material) Compared of with high-refractive-index dielectric thinoffilms, the low-refractive-index dielectric was thin considered films seem to be seldom used for the performance improvement of an SPR In this[18]. paper, we because its refractive index can change from 1.05 to 1.46 by controlling itssensor. porosity Besides having a studied the effect of a low-refractive-index dielectric thin film on the performance of an SPR sensor, low-refractive-index, porous silica film has several other advantageous properties, such as low thermal which has not been reported in literature yet. conductivity, high mechanical and excellent stability material) [19]. These characteristics make As an example of astrength, study, porous silica (a kind ofthermal low-refractive-index was considered because its refractive index can change from 1.05 to 1.46 by controlling its porosity [18]. it applicable to many optical aspects, such as the preparation of antireflection coatingsBesides [20]. Particularly, having a low-refractive-index, porous silica film has several other advantageous properties, such as porous silicalow film has also been employed for performance improvement of an SPR sensor by reducing thermal conductivity, high mechanical strength, and excellent thermal stability [19]. These its FWHM ofcharacteristics the reflectance dip [21]. make it applicable to many optical aspects, such as the preparation of antireflection coatings [20]. be Particularly, porous silica in filmthis has paper also beentoemployed fordetail performance improvement Therefore, it will of much interest study in the effect of a porous silica of an SPR sensor by reducing its FWHM of the reflectance dip [21]. film on the performance of an SPR sensor, and to explore the possibility of FOM enhancement by using Therefore, it will be of much interest in this paper to study in detail the effect of a porous silica porous silicafilm film. theofdiffusion of analyte fluid the intopossibility the porous silica film, which on To the prevent performance an SPR sensor, and to explore of FOM enhancement by may affect using porous silica film. To prevent the diffusion of analyte fluid into the porous silica film, which its refractive index, a thin SiO2 film was used as a protective film at top of the SPR sensor (as shown affect its refractive index, a thin SiO2 film was used as a protective film at top of the SPR sensor in Figure 1).may The obtained result indicated that by adding a 40 nm thick porous silica film with the (as shown in Figure 1). The obtained result indicated that by adding a 40 nm thick porous silica film porosity of 90% to the refractive of 1.04) on1.04) toponoftop theof metal film, with (corresponding the porosity of 90% (corresponding to theindex refractive index of the metal film,the the FOM can be be enhanced byof an 311%, increase compared factor of 311%, compared of a traditional sensor. enhanced byFOM an can increase factor with that with of a that traditional SPRSPR sensor.
Figure 1. Schematic the surface plasmon resonance resonance (SPR) sensor with a porous film. silica film. Figure 1. Schematic of the ofsurface plasmon (SPR) sensor with asilica porous
2. Calculation Methods
2. Calculation Methods 2.1. Three-Layer Model
To theoretically analyze the performance of an SPR sensor, we employed a typical thin-film theory [22], which has been widely validated with real SPR systems [23,24]. The SPR sensor with a
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2.1. Three-Layer Model
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To theoretically analyze the performance of an SPR sensor, we employed a typical thin-film theory [22], which has been widely validated with real SPR systems [23,24]. The SPR sensor with a porous silica asaathree-layer three-layer model shown in Figure 2), which contains porous silicafilm filmcan canbe be treated treated as model (as (as shown in Figure 2), which contains a metal a metal film, a poroussilica silicafilm, film, and and aasilica prism andand analyte are considered as the as incident and film, a porous silicafilm. film.The The prism analyte are considered the incident and emergent media respectively. emergent media respectively.
Figure2.2.Three-layer Three-layer model of the proposed SPR sensor. Figure model of the proposed SPR sensor.
2.2.2.2. Reflectance ReflectanceCoefficient Coefficient For this three-layer model, tangential components of the electric magnetic For this three-layer model, thethe tangential components of the electric and and magnetic fieldfield amplitudes amplitudes at interface 1 (E 1 and H 1 ) are connected to those at interface 4 (E 4 and H 4) by the at interface 1 (E1 and H1 ) are connected to those at interface 4 (E4 and H4 ) by the characteristic characteristic matrix [22]. This is written as matrix [22]. This is written as " # " #" #" #" # ii i i i sinδps i cos δm cos δ cos δ sin δ sin δ E1 E ps s m s 4 η η η cos ps sin ps m sin cos s sin s E4 E= cos m , (1) m ps cospsδps s δs s cos H1 1 iηm sin δm mcos δm iηps sin δps H iηs sin (1) 4 δs ， H H1 sin cos ips sin ps cos ps is sin s cos s 4 mand η m (i = m, m where δi (i = m, ps, is) ps, s) are the phase factor and the optical admittance of the metal i
(m), porous silica δi = di cosθ in which ni and di respectively represent the i /λ,factor where δi (isilica = m,(ps), ps, s)orand ηi (s) (i =film. m, ps, s) 2πn are ithe phase and the optical admittance of the refractive index and thickness of each film. λ is the wavelength of the incident light, and θ represents i metal (m), porous silica (ps), or silica (s) film. δi = 2πnidicosθi/λ, in which ni and di respectively therepresent propagation angle inindex each and medium. η i =ofnieach /cosθ for p-polarized the refractive thickness film. λ is the wavelength of the light. incident light, i is satisfied and θi represents angle in each medium. i = ni/cosθand i is satisfied light. optical Because H4 /Ethe η a (the optical admittance of theηanalyte) H1 /E1 for = Yp-polarized (the equivalent 4 =propagation Because H4/Eassembly), 4 = ηa (the optical admittance thebe analyte) and H admittance of the Equation (1) can of also expressed as1/E1 = Y (the equivalent optical " # of"the assembly), Equation #" #" # admittance (1) can also be expressed as #" i i i cos δps cos δm cos δs 1 1 ηps sin δps ηm sin δm ηs sin δs E1 = E4 . Y ηa i δm δs iηcos iηm sin δm cos iηs sin δi s cos δps i cos δps ps sin sin cos sin cos sin 1 ps ps s s 1 m m s m 1 E4 . So, theEcharacteristic matrix of the assembly is ps Y (2) cos m #" cos ps is#" sin s cos s a #" ps " # " im sin m # ips sin i i i cos δps cos δm cos δs B 1 ηps sin δps ηm sin δm ηs sin δs = . C ηa iηm sin δm cos δm iηs sin δs cos δs iηps sin δps cos δps So, the characteristic matrix of the assembly is
(2)
(3)
The equivalent admittance Y can be calculated through Y = C/B, and the reflectance R of the SPR i sensor is i i sin m cos ps η −sin B cos m Y ps cos s sin s 1 . p m ps (3) s R = (4) , C i sin +Y cos m ips sin ps ηp cos ps is sin s cos s a m m where ηThe the optical admittance of be thecalculated prism. through Y = C/B, and the reflectance R of the SPR p isequivalent admittance Y can sensor is
2.3. Performance Parameters The performance of an SPR sensor can be described by several parameters, such as the resonance angle θ res , depth of dip Rres (the reflectance at the resonance angle), sensitivity, FWHM, and FOM. In the angular interrogation mode, the resonance angle θ res changes with the refractive index of the analyte ns , so the sensitivity S can be expressed as the ratio between the resonance angle variation ∆θ res , and the variation of refractive index of the analyte ∆ns as follows [5]: S=
∆θres . ∆ns
(5)
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The FWHM can be determined by calculating the full width at half maximum of the reflectance dip (∆θ 0.5 ), expressed as FWHM = ∆θ0.5 . (6) Thus, the value of the FOM can be calculated by FOM =
S . FWHM
(7)
3. Results and Discussions The performance of a prism-based SPR sensor is generally determined by the wavelength and incident angle of the light source, the material and thickness of each film used in the SPR sensor, and the material of the prism and analyte [25]. In our simulations, a laser with 632.8 nm wavelength was taken as the incident light source. Silver (Ag) was applied as the metal material in the SPR sensor (nm = 0.13 + 3.99i), which can provide sharper reflectance dip than gold (Au) [21,26]. The thickness of the Ag film was assumed to be 47 nm to ensure an approximate-zero reflectance at the resonance angle. As mentioned above, a 10 nm thick SiO2 film was employed as the protective film (ns = 1.46). In addition, a 45-degree SF-11L glass was used as the prism (np = 1.73205) in the SPR sensor [27], and water was considered as the analyte (na = 1.33) in our simulations. 3.1. The Refractive Index of Porous Silica To determine the effect of a porous silica film on the performance of an SPR sensor, the characteristic of the refractive index of the porous silica film needs to be studied in advance. The two-component Bruggeman approximation model [20] can be adopted to express the relationship between the refractive index and the porosity for porous silica film, which is written as P
n2 s − n2 ps 1 − n2 ps = 0, + ( 1 − P ) 1 + 2n2 ps n2 s + 2n2 ps
(8)
where P is the porosity of the porous silica film, and ns and nps are the refractive index of silica and porous silica, respectively. In this way, based on Equation (8), the relationship between the refractive index and Sensors 2017, 17,the 1846porosity can be determined, as shown in Figure 3. 5 of 11
Refractive index (RIU)
1.5 1.4 1.3 1.2 1.1 1.0 0.0
0.2
0.4 0.6 Porosity
0.8
1.0
Figure 3. 3. Refractive index as as a function of of thethe porosity ofof thethe porous silica film. Figure Refractive index a function porosity porous silica film.
3.2. The Effect of of Porous Silica Film onon thethe SPR Sensor 3.2. The Effect Porous Silica Film SPR Sensor InInour and 1.086, ourstudy, study,the theporous poroussilica silicafilms filmswith witha arefractive refractiveindex indexofof1.365, 1.365,1.270, 1.270,1.176, 1.176, and 1.086, corresponding to 20%, 40%, 60%, and 80% porosity respectively, were chosen to explore the effect corresponding to 20%, 40%, 60%, and 80% porosity respectively, were chosen to explore the effect of of a a porous silica film on the performance of an SPR sensor. porous silica film on the performance of an SPR sensor. 3.2.1. Porous Silica with 20% Porosity (nps = 1.365) Firstly, we presented the effect of a porous silica film with 20% porosity (nps = 1.365) on the performance of an SPR sensor. The angular spectra (reflectance versus incident angle) of the SPR sensors with 0 (traditional SPR sensor used for comparison), 20, and 40 nm thick 20% porosity porous silica film were plotted in Figure 4. The performance parameters for each sensor are listed in Table 1.
3.2. The Effect of Porous Silica Film on the SPR Sensor In our study, the porous silica films with a refractive index of 1.365, 1.270, 1.176, and 1.086, corresponding to 20%, 40%, 60%, and 80% porosity respectively, were chosen to explore the effect of a porous film Sensorssilica 2017, 17, 1846on the performance of an SPR sensor. 5 of 11 3.2.1. Porous Silica with 20% Porosity (nps = 1.365)
3.2.1. Porous Silica with 20% Porosity (nps = 1.365)
Firstly, we presented the effect of a porous silica film with 20% porosity (nps = 1.365) on the Firstly, we presented the effect of a porous silica film with 20% porosity (nps = 1.365) on the performance of an SPR sensor. (reflectanceversus versus incident angle) of SPR the SPR performance of an SPR sensor.The Theangular angular spectra spectra (reflectance incident angle) of the sensors withwith 0 (traditional SPR sensor 20,and and4040 nm thick 20% porosity porous sensors 0 (traditional SPR sensorused usedfor for comparison), comparison), 20, nm thick 20% porosity porous silicasilica filmfilm were plotted in in Figure 4.4.The parametersfor foreach each sensor listed in Table were plotted Figure Theperformance performance parameters sensor areare listed in Table 1. 1. 1.0
Reflectance
0.8 0.6 0.4 0.2
dps=0 nm dps=20 nm; P=20%
0.0 50
dps=40 nm; P=20%
52
54
56
58
60
Incident angle (deg.) Figure 4. The angular spectra ofof the (traditionalSPR SPR sensor), 40 thick nm thick Figure 4. The angular spectra theSPR SPRsensors sensors with with 00 (traditional sensor), 20,20, andand 40 nm 20% 20% porosity porous silica film (n(n psps == 1.365). porosity porous silica film 1.365). performance parametersofofthe theSPR SPR sensors sensors with SPR sensor), 20, and 40 nm Table 1. The Table 1. The performance parameters with00(traditional (traditional SPR sensor), 20, and 40 nm thick 20% porosity porous silica film (n = 1.365). ps thick 20% porosity porous silica film (nps = 1.365). Thickness of Porous Thickness of Porous Resonance Resonance Depth of Dip (°/RIU) Depth of Dip Sensitivity Sensitivity (◦ /RIU) SilicaSilica Film Film (nm)(nm) Angle (°) (◦ ) Angle 0 54.650 3.273.27 × 10×−410−4 67.9 0 54.650 67.9 20 20 55.715 3.583.58 × 10×−410−4 53.7 55.715 53.7 55.974 43.8 43.8 40 40 55.974 3.653.65 × 10×−410−4
FWHM(◦(°) FWHM )
−1 ) −1) FOMFOM (RIU(RIU
1.513 1.513 1.751 1.751 1.784 1.784
44.888 44.888 30.654 30.654 24.550 24.550
Compared with a traditional poroussilica silica film, 0 nm), the SPR Compared with a traditionalSPR SPRsensor sensor (without (without porous film, i.e.,i.e., dps d=ps0=nm), the SPR sensor with a 20nm nm (or thick 20%20% porosity porousporous silica filmsilica exhibits a slightly larger resonance sensor with a 20 (or 40 40nm) nm) thick porosity film exhibits a slightly larger − 3 ◦ −3 angle and depth of depth dip (butofRres < 1(but × 10Rres).
