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Long-range surface plasmon resonance sensors fabricated with plasma polymerized fluorocarbon thin films
Sensors and Actuators B 215 (2015) 368–372
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Long-range surface plasmon resonance sensors fabricated with plasma polymerized fluorocarbon thin films Lei Wang, Xiao-Jun Liu, Jie Hao, Li-Qiang Chu ∗ Tianjin University of Science and Technology, No. 29, 13th Ave., TEDA, Tianjin 300457, China
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Article history: Received 4 December 2014 Received in revised form 25 February 2015 Accepted 2 April 2015 Available online 15 April 2015 Keywords: Long-range surface plasmon resonance (LRSPR) Optical waveguide spectroscopy (OWS) Plasma polymerization Fluorocarbon films Perfluorooctyl ethylene (PFOE)
a b s t r a c t A rapid and simple approach to prepare the dielectric buffer layers is of crucial importance for the development of long-range surface plasmon resonance (LRSPR) sensors. In this regard, we describe for the first time the use of plasma polymerized fluorocarbon thin films as the dielectric buffer layers for the construction of LRSPR sensors. The fluorocarbon films were achieved by radio frequency plasma polymerization of perfluorooctyl ethylene (PFOE) at continuous wave mode with an input power of 60 Watts. The resulting ppPFOE exhibited good adhesion with both the glass substrate and the gold superstrate, and also remained relatively stable in aqueous solutions as seen by optical waveguide spectroscopy (OWS). The obtained LRSPR sensor consisting of a SF11-ppPFOE-Au structure was employed for the detection of both the bulk refractive index variation and the protein adsorption. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently long-range surface plasmon resonance (LRSPR) has attracted considerable attention because of their potential applications for cellular studies [1–3] and the detection of pathogens [4–6]. Compared with conventional surface plasmon resonance (cSPR) in the Kretschmann’s configuration, LRSPR exhibits a stronger surface electric field strength, a narrower resonance dip, as well as, a longer penetration depth up to several micrometers [7–11]. Consequently, a LRSPR sensor has an extended detection range from metallic surface compared to a cSPR sensor. The multilayered structure required for the excitation of LRSPR usually contains an optimum thickness of dielectric (so-called buffer layer) embedded between glass substrate and thin metallic layer (e.g., Au) [12–14]. The refractive index of the dielectric buffer layer should be identical/similar to that of sensing medium (e.g., water) in order to obtain a symmetric coupling of the surface plasmon modes on both surfaces of thin metallic layer [15–17]. When taking the refractive index and film thickness control into account, the selection of dielectric buffer materials is limited to several fluoride-containing materials, including evaporated magnesium fluoride (MgF2 ) [14,18], sputtered polytetrafluoroethylene
∗ Corresponding author. Tel.: +86 22 60602476; fax: +86 22 60602430. E-mail address: [email protected] (L.-Q. Chu). http://dx.doi.org/10.1016/j.snb.2015.04.005 0925-4005/© 2015 Elsevier B.V. All rights reserved.
(PTFE) [19,20], as well as, spin-coated fluoropolymers [15,21,22]. Since MgF2 is harmful to human health and has trace solubility in aqueous media, most LRSPR sensors are fabricated by spin-coating two commercially available fluoropolymers: (1). Teflon AF-1600 (DuPont, USA), a polytetrafluoroethylene copolymer with a refractive index nd = 1.3065 at = 632.8 nm [14,15]; (2). Cytop (Asahi Glass Co. Japan), a polydecafluoroxaheptadiene with a refractive index nd = 1.3368 at = 632.8 nm [21,22]. Both fluoropolymers are soluble in the specific solvents provided by the corresponding suppliers. The film thickness (d) can be tuned by adjusting the spin-coating speed and the solution concentration. In order to promote the adhesion of the fluoropolymers onto glass substrates, the glasses should be coated firstly with a fluorosilane solution [7]. After spin-coating the fluoropolymer, the thin film needs to be baked in a series of steps at elevated temperatures in order to remove the specific solvent [14,21]. Therefore the whole fabrication process contains multi-steps, and is time-consuming and tedious, which limits the wide application of LRSPR sensor. An alternative approach to prepare fluorocarbon thin film is plasma polymerization (pp) of various fluoriated precursors [23–25]. Some merits associated with plasma polymerization technique include simple operation, one-step and solvent-free process, easy control of film thickness, good adhesion onto various substrates, etc. Therefore, plasma polymerization of perfluorooctyl ethylene (PFOE) was carried out in this work in order to obtain a fluorocarbon thin film with good adhesion onto the glasses. PFOE
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is chosen here as the precursor because it has a sufficient vapor pressure, thus avoiding the need of carrier gases and making the deposition process easy. The multilayered structure supporting LRSPR was prepared by plasma polymerization of PFOE directly onto the glass slides, and followed by the deposition of a thin Au layer using a thermal evaporator. The thickness of the ppPFOE film was controlled by adjusting the deposition time. Optical waveguide spectroscopy (OWS) was employed to determine both the thickness and the refractive index of the ppPFOE. The stability of the ppPFOE in water was also investigated since LRSPR sensors are supposed to be used in aqueous media. The obtained LRSPR sensor was used for the investigation of bulk refractive index variation and protein adsorption as compared with cSPR. 2. Experimental 2.1. Materials and substrates Perfluorooctyl ethylene (97%) was purchased from SigmaAldrich (Shanghai, China), and was degassed by three freeze-thaw cycles before use, without any further purification. Deionized water was used throughout the experiments. Ethylene glycol (99%) was obtained from Aladdin (Shanghai, China). Lysozyme was purchased from Energy Chemical Co., China. The glass substrates used for all SPR, OWS and LRSPR measurements were 25 × 25 mm SF11 slides (ng = 1.7786 at = 632.8 nm), which were cleaned using 2% Hellmanex solution, rinsed with copious deionized water, and dried with pure nitrogen gas. For SPR and OWS measurements, the SF11 slides were coated with approximately 2 nm of Cr and 52 nm of Au (or Ag), which were thermally evaporated. For the construction of LRSPR sensors, a plasma polymerized fluorocarbon films was deposited directly on a SF11 slides without any pretreatments, thereafter an Au film of 21.9 nm thick was thermally evaporated onto the fluorocarbon layers. 2.2. Plasma deposition of ppPFOE Deposition of ppPFOE films was carried out in a custom-built, capacitively coupled radio frequency (13.56 MHz) plasma reactor as described before [26,27]. The tubular Pyrex glass chamber, enclosed in a Faraday cage, was 40 cm in length and 11 cm in diameter. The plasma power was generated by a plasma power generator, which passed through a matching unit, and was delivered to the reactor via a coil around the exterior of glass tube. The reaction chamber was evacuated down to 6 × 10−3 mbar using a rotary pump. A baratron was connected to the chamber in order to monitor its pressure. A side arm at reactor inlet allowed for the introduction of PFOE vapors. The monomer pressure during plasma deposition was ca. 0.1 mbar. The plasma depositions were carried out at a continuous wave (CW) mode with an input power of 60 W. The deposition time was adjusted to obtain the desired thickness. 2.3. LRSPR, SPR and OWS measurements LRSPR, SPR and OWS measurements were carried out in a custom-built SPR setup based on the Kretschmann configuration, which had been described before [28–30]. A He-Ne laser ( = 632.8 nm, laser power < 5 mW) was used as light source. The light beam passed through two polarizers and a chopper, and then was reflected from the prism base, which was detected by a photodiode and a lock-in amplifier. While LRSPR and SPR could be excited only by p-polarized light (TM polarisation), both p- and s-polarized (TE polarization) were employed for OWS measurements. In contrast to SPR, OWS allowed for the simultaneous determination of both the refractive index and the thickness of plasma polymerized
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films, provided that two waveguide modes were excited. All samples were attached to a flow cell for liquid exchange. A peristaltic pump was employed to introduce different liquids into the flow cell at a flow rate of 1 ml/min. The LRSPR, SPR and OWS spectra (i.e., the reflectivity vs. incident angle curve) could be fitted using the WASPLAS software, which was based on the Fresnel’s multireflection theory and developed by Max-Planck-Institute for Polymer Research, Mainz, Germany. The figure of merit (FOM) was calculated using the following equation according to the literature [8,15]: FOM =
res . FWHM × ns
Here res is the angular shift of the resonance dip. FWHM is the full width at half maximum (FWHM) of the resonance dip, and ns for the bulk refractive index change on sensor surface. The protein density on the surface is calculated using de Feijter’s equation [31,32]: M = dA ×
nA − ns dn/dc
Here, dA and nA are the thickness and the refractive index of the adsorbed protein layer, respectively. ns is the refractive index of a surrounding medium on a metal surface. dn/dc is the refractive index increment, which is equal to approximately 0.182 cm3 /g for proteins [32]. The refractive index of lysozyme is nA = 1.480 as reported previously [32]. 3. Results and discussion When used as the dielectric buffer layer in a LRSPR sensor, the thin film should fulfill several requirements, including a suitable refractive index, an optimum thickness, a low surface roughness, as well as, a good adhesion with substrates. In biosensor applications, the sensor architectures should also show sufficient stability in aqueous medium. Therefore, based on our experiences about plasma polymerization of PFOE, the ppPFOE films used in the study were prepared at a CW mode with an input power of 60 Watts. 3.1. Refractive index and thickness of ppPFOE films OWS is a powerful optical technique for the analysis of thin films [28]. When at least two waveguide modes are observed in both pand s-polarized OWS spectra, one can obtain the thickness and the refractive index simultaneously from the fitting of the OWS spectra. Fig. 1A gives the p-poalrized OWS spectra of a series of ppPFOE films at different deposition time. Five sample positions were measured at ambient atmosphere at room temperature. The standard deviation originated from different positions is lower than 1.5%. From Fig. 1A, it is apparent that, when increasing deposition time from 5 to 8 min, the waveguide mode shifts to a higher angle. When further increasing the deposition time to 10 min, a new waveguide mode appears, and eventually the OWS spectrum of 23 min deposition shows five waveguide modes. The OWS spectra measured with s-polarized light also confirm the similar trend (data not show here). After the fitting of OWS spectra with WASPLAS program (based on Fresnel’s equation), one can find that the ppPFOE thicknesses are proportional to the deposition time, as shown in Fig. 1B. The calculated deposition rate is 68.4 nm/min with a linearity r = 0.995. These results indicate that plasma polymerization method allows for a good control over the film thickness. The refractive indices of various ppPFOE films are also shown in Fig. 1A. The average refractive index of ppPFOE films in air is nd = 1.3756 ± 0.0022. The refractive index value is consistent with the spectroscopic ellipsometer result of nd = 1.3710 (data not shown here). The ppPFOE’s refractive index
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Fig. 2. Evolution of the thickness and the refractive index of a 1654.0 nm ppPFOE in water as measured by OWS. Plasma deposition conditions: CW, 60 W, 0.1 mbar.
deviation for the refractive index is only 0.01%. Therefore, the ppPFOE deposited at CW, 60 W is relatively stable in aqueous media, which might be attributed to the hydrophobic nature of the ppPFOE and a high cross-linking degree under the CW plasma. Notice that the thickness of the freshly deposited ppPFOE in air is d = 1654.0 nm (nd = 1.3774) (as indicated in Fig. 1A). When brought in contact with water, its thickness increases slightly to d = 1678.0 nm (nd = 1.3783) (Fig. 2). The increase in both the thickness and the refractive index can be attributed to the uptaking of H2 O molecules by ppPFOE when the surrounding medium changed from air to water. 3.3. Characteristics of LRSPR
Fig. 1. (A) p-polarized OWS spectra of the ppPFOE films obtained at various deposition time; (B) Calculation curve of the ppPFOE deposition rate. The solid line is the fitting curve with a linearity of r = 0.995. Plasma deposition conditions: CW, 60 W, 0.1 mbar.
is lower than that of MgF2 (nd = 1.38) [14], suggesting that it should be useful for the construction of LRSPR sensors. 3.2. Stability of ppPFOE in water It is known that plasma polymerized thin films, especially those deposited at a low input power, contain some non-cross-linked materials, which can be washed away in liquid media [27,33]. The loss of materials will result in a significant decrease in film thickness. Therefore, the stability of ppPFOE films in water was investigated by using OWS. The ppPFOE of 1654.0 nm thick was used for this purpose because it could exhibit two waveguide modes in water. The OWS spectra were recorded at different time using both p- and s-polarized light (data not shown here). Fig. 2 shows the fitting results about the film thickness and the refractive index in water at different time. One can find that, after 8 h immersion in water, both the ppPFOE thickness and the refractive index increase slightly. The absolute thickness change in water is d = 23.7 nm, which is about 1.4% with respect to the total fresh ppPFOE. The refractive index change is only 0.0009. Those increases might be explained by the penetration of H2 O molecules into the plasma polymer matrix, in which some free volume might exist and could be occupied by H2 O molecules. The average refractive index of the ppPFOE in water is nd = 1.3785 ± 0.0004, which will be used for the fitting of LRSPR spectrum later. The calculated standard
The multilayered structure supporting LRSPR was successfully obtained by sequential deposition of a ppPFOE and a thin Au layer without any other surface treatments. No delamination was observed for either the Au layer or the ppPFOE coatings, indicating that the plasma polymerized fluorocarbon film had good adhesion with both the glass substrate and the gold layer. Therefore, the current fabrication process for LRSPR sensor is straightforward and robust. The thickness of Au layer was measured by a quartz crystalline chip during thermal evaporation. The optical parameters for Au layer were obtained from the fitting of the angular reflectance curve for bare Au on a glass substrate. It should be noticed that the complex refractive index of an ultrathin Au layer deviates considerably from the bulk Au, especially when the Au layer’s thickness is lower than 25 nm because the Au layer becomes increasingly lossy [8,13]. On the other hand, since the thickness of plasma polymerized film is dependent on the sample position inside a plasma chamber, it is not possible to deposit the identical thickness of ppPFOE films for both OWS and LRSPR measurements. Hence, the thickness of the ppPFOE was tuned in order to achieve the best fitting of LRSPR spectrum. Ethylene glycol (EG) aqueous solutions of different concentrations were introduced into the flow cell in order to adjust the bulk refractive index on the sensor surface. For all the angular reflectivity curves in Fig. 3, critical angles ( c ) can be observed, which indicate the transition from refraction to total internal reflection at the prism/ppPFOE interface. The refractive indices of various EG solutions were deduced from the critical angles of the corresponding LRSPR spectra (as given in Fig. 3). Similar to SPR, the excitation of LRSPR is verified as a sharp dip in the angular reflectivity curve R(). Fig. 3 compares the reflectivity spectra of a LRSPR sensor with a 690.1 nm of ppPFOE film (nd = 1.3785) and a 21.9 nm of Au layer (nm = 0.2415 + 3.5197i) in various EG solutions. Above the critical angles, narrow resonance
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θ
Fig. 3. LRSPR spectra in various EG solutions of different concentrations. The refractive indices of different EG solutions are also given. The LRSPR sensor consists of a 690.1 nm of ppPFOE and a 21.9 nm of Au layer.
Fig. 5. Comparison of LRSPR and cSPR spectra with respect to lysozyme adsorption on bare Au surface. The LRSPR sensor consists of a 740.0 nm of ppPFOE and a 21.9 nm of Au layer. The SPR sensor is with a 2 nm of Cr and a 52 nm of Au.
3.4. Comparison of LRSPR to cSPR dips ( res ) can be observed, which clearly reveal the generation of LRSPR in the current SF11-ppPFOE-Au architecture. When changing the EG solution from 22% to 23%, 24% and 25%, the resonance dips res of LRSPR shift to a higher angle accordingly. In angular modulated SPR, the sensor sensitivity can be calculated by a change of the measured res due to a unity refractive index change. From Fig. 3, the FWHM of a LRSPR spectrum (e.g., ns = 1.3525) is 0.677 degree. The res is 0.050 degree when the solution refractive index change from ns = 1.3525 to 1.3533. Hence, a figure of merit was determined to be FOM = 92.3 RIU−1 , which is lower that the reported highest value of FOM = 466 RIU−1 [15], but still 3.8-folds higher than cSPR with a FOM = 24.3 RIU−1 [15]. The current LRSPR sensor is comparable with that with a 895 nm Cytop and a 22.5 nm Au in the literature [8]. Fig. 4 shows the realtime monitoring of the bulk refractive index variation by using the current LRSPR sensor. It can be found that the response of LRSPR sensor to liquid change is very prompt and then becomes stable quickly.
In order to compare the performance of LRSPR sensors with cSPR, we measured the protein adsorption behavior on bare gold. Lysozyme was chosen as model protein. The molecule weigh of lysozyme is about 14.3 kDa with a size of 4.5 nm × 3.0 nm × 3.0 nm [34]. A dense layer of lysozyme thus leads to a theoretic surface density ranging from 176.0 to 263.9 ng/cm2 . PBS buffer of pH = 7.4 was used for protein adsorption experiment, in which lysozyme became positively charged. Fig. 5 compares the LRSPR and cSPR spectra for the detection of lysozyme adsorption onto bare gold surface. Since the refractive index of lysozyme is equal to nA = 1.480 [32], the thickness of adsorbed lysozyme could be obtained after the fitting, which were 3.2 nm and 3.4 nm for LRSPR and cSPR, respectively. The surface density of lysozyme on the Au surface can be calculated by using de Feijter’s equation [31,32]. For LRSPR and cSPR sensor, the surface densities of adsorbed lysozyme are 254.2 and 270.1 ng/cm2 , respectively, which clearly suggest the formation of a dense lysozyme monolayer on the Au surface. From Fig. 5, one can find that the FWHM of the LRSPR spectrum is 0.789 degree, which is much narrower compared with that of a cSPR spectrum (i.e., FWHM = 4.595 degree). This can be ascribed to a lower loss in LRSPR. The result is in good agreement with the literature [8]. Considering the fact that the ns for lysozyme adsorption is not possible to obtain, we would not calculate the FOM values for both LRSPR and cSPR sensor, respectively. On the other hand, in order to compare the sensor sensitivity for surface refractive index change, the ratio between the FOMLRSPR and FOMcSPR is then calculated, which is equal to FOMLRSPR /FOMcSPR = 1.1. Dostálek et al. [8] showed that the sensitivity enhancement with a Cytop LRSPR sensor in the best case was 2.4-folds higher than cSPR. The surface sensitivity of the current LRSPR sensor is apparently lower than that value [8,21]. Therefore, further improvements in the construction of LRSPR sensors, including the deposition of the ppPFOEs and the Au layers, are still highly desired, which will be the scope of our future work. 4. Conclusions
Fig. 4. LRSPR real-time monitoring of the bulk refractive index variation at a fixed incidence angle of 55.5 degree (see Fig. 3).
In this contribution we demonstrate the use of ppPFOE film (CW, 60 W) as dielectric buffer layer for the construction of LRSPR sensors. Plasma techniques are employed to improve the surface energy of spin-coated fluoropolymers previously [15,22]. However,
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to the best of our knowledge, this is the first report that plasma polymerization technique allows for the fabrication of dielectric buffer layers. The resulting LRSPR sensors exhibit a FOM = 92.3 RIU−1 with respect to the detection of bulk refractive index variation, which is 3.8-folds higher than that of a cSPR sensor. The ppPFOE film (CW, 60 W) has good adhesion onto both the glass substrate and the gold layer, thus avoiding the pre- and post-treatment processes. Moreover, plasma polymerization is well established in industry and is an environmental-benign technique without the need of any solvents. Therefore, compared to spin-coating method, this fabrication process appears more appropriate and promising for the large scale production and wide application of LRSPR sensors. Acknowledgements The authors thank financial support by Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 12JCYBJC31700) and Program for New Century Excellent Talents in University, NCET-12-1064. References [1] V. Chabot, Y. Miron, M. Grandbois, P.G. Charette, Long range surface plasmon resonance for increased sensitivity in living cell biosensing through greater probing depth, Sens. Actuators B 174 (2012) 94–101. [2] M. Vala, R. Robelek, M. Bockova, J. Wegener, J. Homola, Real-time label-free monitoring of the cellular response to osmotic stress using conventional and long-range surface plasmons, Biosens. Bioelectron. 40 (2013) 417–421. [3] R. Mejard, H.J. Griesser, B. Thierry, Optical biosensing for label-free cellular studies, TrAC-Trends Anal. Chem. 53 (2014) 178–186. [4] M. Vala, S. Etheridge, J.A. Roach, J. Homola, Long-range surface plasmons for sensitive detection of bacterial analytes, Sens. Actuators B 139 (2009) 59–63. [5] Y. Wang, W. Knoll, J. Dostalek, Bacterial pathogen surface plasmon resonance biosensor advanced by long range surface plasmons and magnetic nanoparticle assays, Anal. Chem. 84 (2012) 8345–8350. [6] C.J. Huang, W. Knoll, A. Sessitsch, J. Dostalek, SPR bacterial pathogen biosensor: the importance of fluidic conditions and probing depth, Talanta 122 (2014) 166–171. [7] R. Slavik, J. Homola, Ultrahigh resolution long range surface plasmon-based sensor, Sens. Actuators B 123 (2007) 10–12. [8] J. Dostalek, A. Kasry, W. Knoll, Long range surface plasmons for observation of biomolecular binding events at metallic surfaces, Plasmonics 2 (2007) 97–106. [9] J. Dostalek, R.F. Roskamp, W. Knoll, Coupled long range surface plasmons for the investigation of thin films and interfaces, Sens. Actuators B 139 (2009) 9–12. [10] P. Berini, Long-range surface plasmon polaritons, Adv. Opt. Photon. 1 (2009) 484–588. [11] C.J. Huang, J. Dostalek, W. Knoll, Long range surface plasmon and hydrogel optical waveguide field-enhanced fluorescence biosensor with 3D hydrogel binding matrix: On the role of diffusion mass transfer, Biosens. Bioelectron. 26 (2010) 1425–1431. [12] D. Sarid, Long-range surface-plasma waves on very thin metal films, Phys. Rev. Lett. 47 (1981) 1927–1930. [13] M.A. Kessler, E.A.H. Hall, Multilayered structures exhibiting long-range surface exciton resonance, Thin Solid Films 272 (1996) 161–169. [14] G.G. Nenninger, P. Tobiska, J. Homola, S.S. Yee, Long-range surface plasmons for high-resolution surface plasmon resonance sensors, Sens. Actuators B 74 (2001) 145–151. [15] R. Mejard, J. Dostalek, C.-J. Huang, H. Griesser, B. Thierry, Tuneable and robust long range surface plasmon resonance for biosensing applications, Opt. Mater. 35 (2013) 2507–2513. [16] N.M. Lyndin, I.F. Salakhutdinov, V.A. Sychugov, B.A. Usievich, F.A. Pudonin, O. Parriaux, Long-range surface plasmons in asymmetric layered metal-dielectric structures, Sens. Actuators B 54 (1999) 37–42. [17] K. Matsubara, S. Kawata, S. Minami, Multilayer system for a high-precision surface plasmon resonance sensor, Opt. Lett. 15 (1990) 75–77.
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Biographies Lei Wang received his bachelor degree from Tianjin University of Science & Technology, China, in June 2013. Currently, he is a Master candidate under Professor Li-Qiang Chu at Tianjin University of Science & Technology, China. His present works are the development of long-range surface plasmon resonance related techniques. Xiao-Jun Liu graduated from Liaoning Technical University, China (2012), majoring in materials science and engineering. Now He is a Master student at Tianjin University of Science & Technology. His research is about the preparation of transparent hydrophobic coatings by plasma chemical vapor deposition. Jie Hao completed his undergraduate at the Tianjin University of Science & Technology in 2013, and now is pursuing his master degree in the same university. His major is material science and his current research interest is plasma polymerization of functional materials. Li-Qiang Chu received his BSc from the Lanzhou University, China (1997) and then worked as a research assistant in Dalian Institute of Chemical Physics, CAS. He obtained his MSc from the National University of Singapore, Singapore (2004) and PhD from Max-Planck-Institute for Polymer Research, Germany (2007). He then worked as a postdoctoral fellow at the University of Notre Dame and the University of California, Davis. He started as a full professor of materials science at Tianjin University of Science and Technology, China (2011). His research interest includes polymeric biomaterials, SPR optics, PECVD and biosensors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Long-range surface plasmon resonance sensors fabricated with plasma polymerized fluorocarbon thin films Lei Wang, Xiao-Jun Liu, Jie Hao, Li-Qiang Chu ∗ Tianjin University of Science and Technology, No. 29, 13th Ave., TEDA, Tianjin 300457, China
a r t i c l e
i n f o
Article history: Received 4 December 2014 Received in revised form 25 February 2015 Accepted 2 April 2015 Available online 15 April 2015 Keywords: Long-range surface plasmon resonance (LRSPR) Optical waveguide spectroscopy (OWS) Plasma polymerization Fluorocarbon films Perfluorooctyl ethylene (PFOE)
a b s t r a c t A rapid and simple approach to prepare the dielectric buffer layers is of crucial importance for the development of long-range surface plasmon resonance (LRSPR) sensors. In this regard, we describe for the first time the use of plasma polymerized fluorocarbon thin films as the dielectric buffer layers for the construction of LRSPR sensors. The fluorocarbon films were achieved by radio frequency plasma polymerization of perfluorooctyl ethylene (PFOE) at continuous wave mode with an input power of 60 Watts. The resulting ppPFOE exhibited good adhesion with both the glass substrate and the gold superstrate, and also remained relatively stable in aqueous solutions as seen by optical waveguide spectroscopy (OWS). The obtained LRSPR sensor consisting of a SF11-ppPFOE-Au structure was employed for the detection of both the bulk refractive index variation and the protein adsorption. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently long-range surface plasmon resonance (LRSPR) has attracted considerable attention because of their potential applications for cellular studies [1–3] and the detection of pathogens [4–6]. Compared with conventional surface plasmon resonance (cSPR) in the Kretschmann’s configuration, LRSPR exhibits a stronger surface electric field strength, a narrower resonance dip, as well as, a longer penetration depth up to several micrometers [7–11]. Consequently, a LRSPR sensor has an extended detection range from metallic surface compared to a cSPR sensor. The multilayered structure required for the excitation of LRSPR usually contains an optimum thickness of dielectric (so-called buffer layer) embedded between glass substrate and thin metallic layer (e.g., Au) [12–14]. The refractive index of the dielectric buffer layer should be identical/similar to that of sensing medium (e.g., water) in order to obtain a symmetric coupling of the surface plasmon modes on both surfaces of thin metallic layer [15–17]. When taking the refractive index and film thickness control into account, the selection of dielectric buffer materials is limited to several fluoride-containing materials, including evaporated magnesium fluoride (MgF2 ) [14,18], sputtered polytetrafluoroethylene
∗ Corresponding author. Tel.: +86 22 60602476; fax: +86 22 60602430. E-mail address: [email protected] (L.-Q. Chu). http://dx.doi.org/10.1016/j.snb.2015.04.005 0925-4005/© 2015 Elsevier B.V. All rights reserved.
(PTFE) [19,20], as well as, spin-coated fluoropolymers [15,21,22]. Since MgF2 is harmful to human health and has trace solubility in aqueous media, most LRSPR sensors are fabricated by spin-coating two commercially available fluoropolymers: (1). Teflon AF-1600 (DuPont, USA), a polytetrafluoroethylene copolymer with a refractive index nd = 1.3065 at = 632.8 nm [14,15]; (2). Cytop (Asahi Glass Co. Japan), a polydecafluoroxaheptadiene with a refractive index nd = 1.3368 at = 632.8 nm [21,22]. Both fluoropolymers are soluble in the specific solvents provided by the corresponding suppliers. The film thickness (d) can be tuned by adjusting the spin-coating speed and the solution concentration. In order to promote the adhesion of the fluoropolymers onto glass substrates, the glasses should be coated firstly with a fluorosilane solution [7]. After spin-coating the fluoropolymer, the thin film needs to be baked in a series of steps at elevated temperatures in order to remove the specific solvent [14,21]. Therefore the whole fabrication process contains multi-steps, and is time-consuming and tedious, which limits the wide application of LRSPR sensor. An alternative approach to prepare fluorocarbon thin film is plasma polymerization (pp) of various fluoriated precursors [23–25]. Some merits associated with plasma polymerization technique include simple operation, one-step and solvent-free process, easy control of film thickness, good adhesion onto various substrates, etc. Therefore, plasma polymerization of perfluorooctyl ethylene (PFOE) was carried out in this work in order to obtain a fluorocarbon thin film with good adhesion onto the glasses. PFOE
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is chosen here as the precursor because it has a sufficient vapor pressure, thus avoiding the need of carrier gases and making the deposition process easy. The multilayered structure supporting LRSPR was prepared by plasma polymerization of PFOE directly onto the glass slides, and followed by the deposition of a thin Au layer using a thermal evaporator. The thickness of the ppPFOE film was controlled by adjusting the deposition time. Optical waveguide spectroscopy (OWS) was employed to determine both the thickness and the refractive index of the ppPFOE. The stability of the ppPFOE in water was also investigated since LRSPR sensors are supposed to be used in aqueous media. The obtained LRSPR sensor was used for the investigation of bulk refractive index variation and protein adsorption as compared with cSPR. 2. Experimental 2.1. Materials and substrates Perfluorooctyl ethylene (97%) was purchased from SigmaAldrich (Shanghai, China), and was degassed by three freeze-thaw cycles before use, without any further purification. Deionized water was used throughout the experiments. Ethylene glycol (99%) was obtained from Aladdin (Shanghai, China). Lysozyme was purchased from Energy Chemical Co., China. The glass substrates used for all SPR, OWS and LRSPR measurements were 25 × 25 mm SF11 slides (ng = 1.7786 at = 632.8 nm), which were cleaned using 2% Hellmanex solution, rinsed with copious deionized water, and dried with pure nitrogen gas. For SPR and OWS measurements, the SF11 slides were coated with approximately 2 nm of Cr and 52 nm of Au (or Ag), which were thermally evaporated. For the construction of LRSPR sensors, a plasma polymerized fluorocarbon films was deposited directly on a SF11 slides without any pretreatments, thereafter an Au film of 21.9 nm thick was thermally evaporated onto the fluorocarbon layers. 2.2. Plasma deposition of ppPFOE Deposition of ppPFOE films was carried out in a custom-built, capacitively coupled radio frequency (13.56 MHz) plasma reactor as described before [26,27]. The tubular Pyrex glass chamber, enclosed in a Faraday cage, was 40 cm in length and 11 cm in diameter. The plasma power was generated by a plasma power generator, which passed through a matching unit, and was delivered to the reactor via a coil around the exterior of glass tube. The reaction chamber was evacuated down to 6 × 10−3 mbar using a rotary pump. A baratron was connected to the chamber in order to monitor its pressure. A side arm at reactor inlet allowed for the introduction of PFOE vapors. The monomer pressure during plasma deposition was ca. 0.1 mbar. The plasma depositions were carried out at a continuous wave (CW) mode with an input power of 60 W. The deposition time was adjusted to obtain the desired thickness. 2.3. LRSPR, SPR and OWS measurements LRSPR, SPR and OWS measurements were carried out in a custom-built SPR setup based on the Kretschmann configuration, which had been described before [28–30]. A He-Ne laser ( = 632.8 nm, laser power < 5 mW) was used as light source. The light beam passed through two polarizers and a chopper, and then was reflected from the prism base, which was detected by a photodiode and a lock-in amplifier. While LRSPR and SPR could be excited only by p-polarized light (TM polarisation), both p- and s-polarized (TE polarization) were employed for OWS measurements. In contrast to SPR, OWS allowed for the simultaneous determination of both the refractive index and the thickness of plasma polymerized
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films, provided that two waveguide modes were excited. All samples were attached to a flow cell for liquid exchange. A peristaltic pump was employed to introduce different liquids into the flow cell at a flow rate of 1 ml/min. The LRSPR, SPR and OWS spectra (i.e., the reflectivity vs. incident angle curve) could be fitted using the WASPLAS software, which was based on the Fresnel’s multireflection theory and developed by Max-Planck-Institute for Polymer Research, Mainz, Germany. The figure of merit (FOM) was calculated using the following equation according to the literature [8,15]: FOM =
res . FWHM × ns
Here res is the angular shift of the resonance dip. FWHM is the full width at half maximum (FWHM) of the resonance dip, and ns for the bulk refractive index change on sensor surface. The protein density on the surface is calculated using de Feijter’s equation [31,32]: M = dA ×
nA − ns dn/dc
Here, dA and nA are the thickness and the refractive index of the adsorbed protein layer, respectively. ns is the refractive index of a surrounding medium on a metal surface. dn/dc is the refractive index increment, which is equal to approximately 0.182 cm3 /g for proteins [32]. The refractive index of lysozyme is nA = 1.480 as reported previously [32]. 3. Results and discussion When used as the dielectric buffer layer in a LRSPR sensor, the thin film should fulfill several requirements, including a suitable refractive index, an optimum thickness, a low surface roughness, as well as, a good adhesion with substrates. In biosensor applications, the sensor architectures should also show sufficient stability in aqueous medium. Therefore, based on our experiences about plasma polymerization of PFOE, the ppPFOE films used in the study were prepared at a CW mode with an input power of 60 Watts. 3.1. Refractive index and thickness of ppPFOE films OWS is a powerful optical technique for the analysis of thin films [28]. When at least two waveguide modes are observed in both pand s-polarized OWS spectra, one can obtain the thickness and the refractive index simultaneously from the fitting of the OWS spectra. Fig. 1A gives the p-poalrized OWS spectra of a series of ppPFOE films at different deposition time. Five sample positions were measured at ambient atmosphere at room temperature. The standard deviation originated from different positions is lower than 1.5%. From Fig. 1A, it is apparent that, when increasing deposition time from 5 to 8 min, the waveguide mode shifts to a higher angle. When further increasing the deposition time to 10 min, a new waveguide mode appears, and eventually the OWS spectrum of 23 min deposition shows five waveguide modes. The OWS spectra measured with s-polarized light also confirm the similar trend (data not show here). After the fitting of OWS spectra with WASPLAS program (based on Fresnel’s equation), one can find that the ppPFOE thicknesses are proportional to the deposition time, as shown in Fig. 1B. The calculated deposition rate is 68.4 nm/min with a linearity r = 0.995. These results indicate that plasma polymerization method allows for a good control over the film thickness. The refractive indices of various ppPFOE films are also shown in Fig. 1A. The average refractive index of ppPFOE films in air is nd = 1.3756 ± 0.0022. The refractive index value is consistent with the spectroscopic ellipsometer result of nd = 1.3710 (data not shown here). The ppPFOE’s refractive index
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Fig. 2. Evolution of the thickness and the refractive index of a 1654.0 nm ppPFOE in water as measured by OWS. Plasma deposition conditions: CW, 60 W, 0.1 mbar.
deviation for the refractive index is only 0.01%. Therefore, the ppPFOE deposited at CW, 60 W is relatively stable in aqueous media, which might be attributed to the hydrophobic nature of the ppPFOE and a high cross-linking degree under the CW plasma. Notice that the thickness of the freshly deposited ppPFOE in air is d = 1654.0 nm (nd = 1.3774) (as indicated in Fig. 1A). When brought in contact with water, its thickness increases slightly to d = 1678.0 nm (nd = 1.3783) (Fig. 2). The increase in both the thickness and the refractive index can be attributed to the uptaking of H2 O molecules by ppPFOE when the surrounding medium changed from air to water. 3.3. Characteristics of LRSPR
Fig. 1. (A) p-polarized OWS spectra of the ppPFOE films obtained at various deposition time; (B) Calculation curve of the ppPFOE deposition rate. The solid line is the fitting curve with a linearity of r = 0.995. Plasma deposition conditions: CW, 60 W, 0.1 mbar.
is lower than that of MgF2 (nd = 1.38) [14], suggesting that it should be useful for the construction of LRSPR sensors. 3.2. Stability of ppPFOE in water It is known that plasma polymerized thin films, especially those deposited at a low input power, contain some non-cross-linked materials, which can be washed away in liquid media [27,33]. The loss of materials will result in a significant decrease in film thickness. Therefore, the stability of ppPFOE films in water was investigated by using OWS. The ppPFOE of 1654.0 nm thick was used for this purpose because it could exhibit two waveguide modes in water. The OWS spectra were recorded at different time using both p- and s-polarized light (data not shown here). Fig. 2 shows the fitting results about the film thickness and the refractive index in water at different time. One can find that, after 8 h immersion in water, both the ppPFOE thickness and the refractive index increase slightly. The absolute thickness change in water is d = 23.7 nm, which is about 1.4% with respect to the total fresh ppPFOE. The refractive index change is only 0.0009. Those increases might be explained by the penetration of H2 O molecules into the plasma polymer matrix, in which some free volume might exist and could be occupied by H2 O molecules. The average refractive index of the ppPFOE in water is nd = 1.3785 ± 0.0004, which will be used for the fitting of LRSPR spectrum later. The calculated standard
The multilayered structure supporting LRSPR was successfully obtained by sequential deposition of a ppPFOE and a thin Au layer without any other surface treatments. No delamination was observed for either the Au layer or the ppPFOE coatings, indicating that the plasma polymerized fluorocarbon film had good adhesion with both the glass substrate and the gold layer. Therefore, the current fabrication process for LRSPR sensor is straightforward and robust. The thickness of Au layer was measured by a quartz crystalline chip during thermal evaporation. The optical parameters for Au layer were obtained from the fitting of the angular reflectance curve for bare Au on a glass substrate. It should be noticed that the complex refractive index of an ultrathin Au layer deviates considerably from the bulk Au, especially when the Au layer’s thickness is lower than 25 nm because the Au layer becomes increasingly lossy [8,13]. On the other hand, since the thickness of plasma polymerized film is dependent on the sample position inside a plasma chamber, it is not possible to deposit the identical thickness of ppPFOE films for both OWS and LRSPR measurements. Hence, the thickness of the ppPFOE was tuned in order to achieve the best fitting of LRSPR spectrum. Ethylene glycol (EG) aqueous solutions of different concentrations were introduced into the flow cell in order to adjust the bulk refractive index on the sensor surface. For all the angular reflectivity curves in Fig. 3, critical angles ( c ) can be observed, which indicate the transition from refraction to total internal reflection at the prism/ppPFOE interface. The refractive indices of various EG solutions were deduced from the critical angles of the corresponding LRSPR spectra (as given in Fig. 3). Similar to SPR, the excitation of LRSPR is verified as a sharp dip in the angular reflectivity curve R(). Fig. 3 compares the reflectivity spectra of a LRSPR sensor with a 690.1 nm of ppPFOE film (nd = 1.3785) and a 21.9 nm of Au layer (nm = 0.2415 + 3.5197i) in various EG solutions. Above the critical angles, narrow resonance
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θ
Fig. 3. LRSPR spectra in various EG solutions of different concentrations. The refractive indices of different EG solutions are also given. The LRSPR sensor consists of a 690.1 nm of ppPFOE and a 21.9 nm of Au layer.
Fig. 5. Comparison of LRSPR and cSPR spectra with respect to lysozyme adsorption on bare Au surface. The LRSPR sensor consists of a 740.0 nm of ppPFOE and a 21.9 nm of Au layer. The SPR sensor is with a 2 nm of Cr and a 52 nm of Au.
3.4. Comparison of LRSPR to cSPR dips ( res ) can be observed, which clearly reveal the generation of LRSPR in the current SF11-ppPFOE-Au architecture. When changing the EG solution from 22% to 23%, 24% and 25%, the resonance dips res of LRSPR shift to a higher angle accordingly. In angular modulated SPR, the sensor sensitivity can be calculated by a change of the measured res due to a unity refractive index change. From Fig. 3, the FWHM of a LRSPR spectrum (e.g., ns = 1.3525) is 0.677 degree. The res is 0.050 degree when the solution refractive index change from ns = 1.3525 to 1.3533. Hence, a figure of merit was determined to be FOM = 92.3 RIU−1 , which is lower that the reported highest value of FOM = 466 RIU−1 [15], but still 3.8-folds higher than cSPR with a FOM = 24.3 RIU−1 [15]. The current LRSPR sensor is comparable with that with a 895 nm Cytop and a 22.5 nm Au in the literature [8]. Fig. 4 shows the realtime monitoring of the bulk refractive index variation by using the current LRSPR sensor. It can be found that the response of LRSPR sensor to liquid change is very prompt and then becomes stable quickly.
In order to compare the performance of LRSPR sensors with cSPR, we measured the protein adsorption behavior on bare gold. Lysozyme was chosen as model protein. The molecule weigh of lysozyme is about 14.3 kDa with a size of 4.5 nm × 3.0 nm × 3.0 nm [34]. A dense layer of lysozyme thus leads to a theoretic surface density ranging from 176.0 to 263.9 ng/cm2 . PBS buffer of pH = 7.4 was used for protein adsorption experiment, in which lysozyme became positively charged. Fig. 5 compares the LRSPR and cSPR spectra for the detection of lysozyme adsorption onto bare gold surface. Since the refractive index of lysozyme is equal to nA = 1.480 [32], the thickness of adsorbed lysozyme could be obtained after the fitting, which were 3.2 nm and 3.4 nm for LRSPR and cSPR, respectively. The surface density of lysozyme on the Au surface can be calculated by using de Feijter’s equation [31,32]. For LRSPR and cSPR sensor, the surface densities of adsorbed lysozyme are 254.2 and 270.1 ng/cm2 , respectively, which clearly suggest the formation of a dense lysozyme monolayer on the Au surface. From Fig. 5, one can find that the FWHM of the LRSPR spectrum is 0.789 degree, which is much narrower compared with that of a cSPR spectrum (i.e., FWHM = 4.595 degree). This can be ascribed to a lower loss in LRSPR. The result is in good agreement with the literature [8]. Considering the fact that the ns for lysozyme adsorption is not possible to obtain, we would not calculate the FOM values for both LRSPR and cSPR sensor, respectively. On the other hand, in order to compare the sensor sensitivity for surface refractive index change, the ratio between the FOMLRSPR and FOMcSPR is then calculated, which is equal to FOMLRSPR /FOMcSPR = 1.1. Dostálek et al. [8] showed that the sensitivity enhancement with a Cytop LRSPR sensor in the best case was 2.4-folds higher than cSPR. The surface sensitivity of the current LRSPR sensor is apparently lower than that value [8,21]. Therefore, further improvements in the construction of LRSPR sensors, including the deposition of the ppPFOEs and the Au layers, are still highly desired, which will be the scope of our future work. 4. Conclusions
Fig. 4. LRSPR real-time monitoring of the bulk refractive index variation at a fixed incidence angle of 55.5 degree (see Fig. 3).
In this contribution we demonstrate the use of ppPFOE film (CW, 60 W) as dielectric buffer layer for the construction of LRSPR sensors. Plasma techniques are employed to improve the surface energy of spin-coated fluoropolymers previously [15,22]. However,
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to the best of our knowledge, this is the first report that plasma polymerization technique allows for the fabrication of dielectric buffer layers. The resulting LRSPR sensors exhibit a FOM = 92.3 RIU−1 with respect to the detection of bulk refractive index variation, which is 3.8-folds higher than that of a cSPR sensor. The ppPFOE film (CW, 60 W) has good adhesion onto both the glass substrate and the gold layer, thus avoiding the pre- and post-treatment processes. Moreover, plasma polymerization is well established in industry and is an environmental-benign technique without the need of any solvents. Therefore, compared to spin-coating method, this fabrication process appears more appropriate and promising for the large scale production and wide application of LRSPR sensors. Acknowledgements The authors thank financial support by Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 12JCYBJC31700) and Program for New Century Excellent Talents in University, NCET-12-1064. References [1] V. Chabot, Y. Miron, M. Grandbois, P.G. Charette, Long range surface plasmon resonance for increased sensitivity in living cell biosensing through greater probing depth, Sens. Actuators B 174 (2012) 94–101. [2] M. Vala, R. Robelek, M. Bockova, J. Wegener, J. Homola, Real-time label-free monitoring of the cellular response to osmotic stress using conventional and long-range surface plasmons, Biosens. Bioelectron. 40 (2013) 417–421. [3] R. Mejard, H.J. Griesser, B. Thierry, Optical biosensing for label-free cellular studies, TrAC-Trends Anal. Chem. 53 (2014) 178–186. [4] M. Vala, S. Etheridge, J.A. Roach, J. Homola, Long-range surface plasmons for sensitive detection of bacterial analytes, Sens. Actuators B 139 (2009) 59–63. [5] Y. Wang, W. Knoll, J. Dostalek, Bacterial pathogen surface plasmon resonance biosensor advanced by long range surface plasmons and magnetic nanoparticle assays, Anal. Chem. 84 (2012) 8345–8350. [6] C.J. Huang, W. Knoll, A. Sessitsch, J. Dostalek, SPR bacterial pathogen biosensor: the importance of fluidic conditions and probing depth, Talanta 122 (2014) 166–171. [7] R. Slavik, J. Homola, Ultrahigh resolution long range surface plasmon-based sensor, Sens. Actuators B 123 (2007) 10–12. [8] J. Dostalek, A. Kasry, W. Knoll, Long range surface plasmons for observation of biomolecular binding events at metallic surfaces, Plasmonics 2 (2007) 97–106. [9] J. Dostalek, R.F. Roskamp, W. Knoll, Coupled long range surface plasmons for the investigation of thin films and interfaces, Sens. Actuators B 139 (2009) 9–12. [10] P. Berini, Long-range surface plasmon polaritons, Adv. Opt. Photon. 1 (2009) 484–588. [11] C.J. Huang, J. Dostalek, W. Knoll, Long range surface plasmon and hydrogel optical waveguide field-enhanced fluorescence biosensor with 3D hydrogel binding matrix: On the role of diffusion mass transfer, Biosens. Bioelectron. 26 (2010) 1425–1431. [12] D. Sarid, Long-range surface-plasma waves on very thin metal films, Phys. Rev. Lett. 47 (1981) 1927–1930. [13] M.A. Kessler, E.A.H. Hall, Multilayered structures exhibiting long-range surface exciton resonance, Thin Solid Films 272 (1996) 161–169. [14] G.G. Nenninger, P. Tobiska, J. Homola, S.S. Yee, Long-range surface plasmons for high-resolution surface plasmon resonance sensors, Sens. Actuators B 74 (2001) 145–151. [15] R. Mejard, J. Dostalek, C.-J. Huang, H. Griesser, B. Thierry, Tuneable and robust long range surface plasmon resonance for biosensing applications, Opt. Mater. 35 (2013) 2507–2513. [16] N.M. Lyndin, I.F. Salakhutdinov, V.A. Sychugov, B.A. Usievich, F.A. Pudonin, O. Parriaux, Long-range surface plasmons in asymmetric layered metal-dielectric structures, Sens. Actuators B 54 (1999) 37–42. [17] K. Matsubara, S. Kawata, S. Minami, Multilayer system for a high-precision surface plasmon resonance sensor, Opt. Lett. 15 (1990) 75–77.
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Biographies Lei Wang received his bachelor degree from Tianjin University of Science & Technology, China, in June 2013. Currently, he is a Master candidate under Professor Li-Qiang Chu at Tianjin University of Science & Technology, China. His present works are the development of long-range surface plasmon resonance related techniques. Xiao-Jun Liu graduated from Liaoning Technical University, China (2012), majoring in materials science and engineering. Now He is a Master student at Tianjin University of Science & Technology. His research is about the preparation of transparent hydrophobic coatings by plasma chemical vapor deposition. Jie Hao completed his undergraduate at the Tianjin University of Science & Technology in 2013, and now is pursuing his master degree in the same university. His major is material science and his current research interest is plasma polymerization of functional materials. Li-Qiang Chu received his BSc from the Lanzhou University, China (1997) and then worked as a research assistant in Dalian Institute of Chemical Physics, CAS. He obtained his MSc from the National University of Singapore, Singapore (2004) and PhD from Max-Planck-Institute for Polymer Research, Germany (2007). He then worked as a postdoctoral fellow at the University of Notre Dame and the University of California, Davis. He started as a full professor of materials science at Tianjin University of Science and Technology, China (2011). His research interest includes polymeric biomaterials, SPR optics, PECVD and biosensors.