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Научни трудове на Съюза на учените в България-Пловдив. Серия В. Техника и технологии, естествен ии хуманитарни науки, том XVI., Съюз на учените сесия "Международна конференция на младите учени" 13-15 юни 2013. Scientific research of the Union of Scientists in Bulgaria-Plovdiv, series C. Natural Sciences and Humanities, Vol. XVI, ISSN 1311-9192, Union of Scientists, International Conference of Young Scientists, 13 - 15 June 2013, Plovdiv.
SPECTRAL-INTERROGATED SPR ON LOW-COST GRATING
Vytautas Astasauskas1 and Georgi Dyankov2
'Kaunas University of Technology, Kaunas, Lithuania 2Faculty of Physics and Engineering Technology, University of Plovdiv "Paissi Hilendarski", Bulgaria e-mail: dyankov@uni-plovdiv.bg
Abstract
Due to its high applicability and commercial value, surface plasmon resonance (SPR) is one of the most actively researched phenomena in modern physics. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics. The application for large scale bio-sensing is related with searching for new configuration that minimizes costs of fabrication of media supporting SPR. Following this logic we have investigated a commercially available gold-type compact disk for the feasibility for SPR excitation. The disc acts as a grating. We have used a spectral interrogation for SPR excitation and have shown that SPR resonance can be tuned in the range 580 nm - 720 nm with different excitation efficiency. The optimal excitation has been chosen at right incident angle. A feasibility of bio-sensor - grating covered with a bio-layers - has been studied. A clear shift of SPR has been observed. The main conclusion of our study is that commercially available, gold-type compact disk can be used as a low-cost gold-coated diffraction grating structure where SPR can be excited with good efficiency.
Introduction
Bearing in mind the technological potential related to the phenomenon SPR and the necessity for low cost media that support plasmons and suitable for large scale and low cost production, we have investigated the possibility for SPR excitation in commercially available gold-type recordable disk (CD-R). This idea has been already exploited [1] and has been shown that goldtype CD is suitable medium for SPR excitation. However, many questions related to the optimal conditions ensuring high sensing accuracy and sensitivity are still open.
The new result, reported in this paper, is a feasibility of gold-type CD for spectral SPR interrogation. The conditions for narrow and deep resonance as a function of wavelength and incident angle have been experimentally studied. We have shown that even in non-optimal conditions of spectral interrogation the structure possesses enough sensitivity and accuracy for bio-sensing.
Experimental set-up
The feasibility of gold-type CD for SPR excitation was studied with the experimental set-up shown in Fig.1.
Fig.1 Experimental set-up;
1 - light source; 2 -power sou rce; 3 - collimator; 4- j)olarizer; 5 - goniometer ; 6 - objective; 7 - spectrometer.
As a light source we have used a halogen lamp or LED. Both sources emi; ucpolarized light. Fig. 2 shows the spectrum of the sources. Depending on the effech we have studied appropriate sourc e was used.
Fig.2 The spectrum of halogen lamp and LED.
The ideal spectrum has smooth shape and LED spectrum is close to it. For this re ason LED lias been used for plasmon excitation in the range 5550-6550 nm However, his full with at half maximum (FWHmis) is only 100 nm an d excitation is not effective. The spectrum of halogen lami) has many dip s at 620 nm, 640 nm, 810 nm and 850 nm what makes SPR obrervation is ambiguous.From other hrnd FWHM of lamp spectrum es about 250 nm what make plasmon excitation more effective.
3. SPR excitation
The conditions for SPR excitation are fulfilled for some range of incident angle and related range of wavelength. The conditions depend on the grating characteristics - period, amplitude and shape of a grating. We have studied experimentally the conditions for SPR excitation. The grating was illuminated by p - polarized white light at different incident angles. The spectral distribution of reflected light was analyzed by a spectrometer. The plasmon resonance is observed as a dip on the source spectrum, as shown in Fig. 3 a. The spectrum of source serves as a reference signal. Usually, the reflectance is used to present the SPR excitation. To follow the adopted convention, 266
the reflectance curves have been calculated. For this purpose the resonance curve is subtracted from reference curve and the absorption is determined. Then, the absorption is converted to the reflectance which is shown in Fig. 3b.
Fig.3 SPR excitation: a-reference and resonance curves; b- reflectance curve
The resonance with low depth and wide FWHM is not effective - small spectral shift cannot be detected. Sensing based on such resonance is with low accuracy and sensitivity. That way it is very important to find the optimal conditions for SPR excitation - at what angle and what wavelength the excited plasmon has narrow and deep resonance. Fig. 4 demonstrates the results of our experimental investigation. Fig. 4b shows the position of the resonance minimums with the fitting curve. Obviously, at small angle of incidence the effectiveness of excitation is higher. The extrapolation data suggests an optimal excitation at angle about 7 deg with 850 nm. Unfortunately, our experimental set-up does not allow an excitation at incident angles smaller than 35 deg. The object of our future experiments will be plasmon excitation at angle close to the normal incidence with the corresponding wavelength.
Fig. 4 Tuning the plasmon resonance in 2D space "wavelength-incident angle"; a - excitation at different angle with the corresponding resonance wavelengths; b - interpolated dependence "angle-wavelength" for the resonance wavelength
3. The feasibility of bio-chip based on golden-type CD-R
The measurements reported in previous sections were performed with bare grating. To check the feasibility of grating - golden CD-R as a bio-chip, two different bio-layers - myoglubin and chitosan - have been immobilized on the grating. Here we report the results related to the chitosan.
Chitosan is a linear polysaccharide. As a film deposited on the metal surface it can serve for detection of Fe, Cu and Pb ions and for controlled attachment of bio-molecules [2,3].
Chitosan was deposited on the grating by spin coating. The thickness of the layer is supposed to be about 50 nm (based on previous experience).
The immobilization of chitosan provokes a clear resonance shift with about 4.5 nm comparing
with SPR resonance on bare grating what is illustrated in Fig. 5.
Fig. 5 SPR sensing of chito-san immobilized on grating when enzyme is deposited on the surface of the grating and then heated
The sensitivity of SPR sensor was checked by detecting the phase change of chitosan which occurs at 47 0C. For this purpose the bio-chip was heated up to 51 0C. The detected shift was about 0.5 nm (compare the lowest and the middle curve in Fig. 5b).
4.Conclusion
The detection of immobilized chitosan was performed for plasmon excitation at 35 deg with radiation at 683 nm. As has been discussed in Sec.3 this excitation is away from the optimal conditions. In spite of this the change of the structure of chitosan, provoked by a heating at temperature 470 C, is clearly detected. This result confirm that SPR sensor based on gold CD-R, which plays a role of grating, possesses accuracy and sensitivity which enable to be used as a bio-sensor.
The optimization of plasmon excitation will increase the performance of the bio sensor.
With this work we show the feasibility of gold-type CD-R as a grating which supports plasmon and posses a potential to increase the performance when the wavelength interrogation is applied.
References
1. E.Fontana, Appl.Opt. 43, 79, 2004.
2. Shenhsiung Lin, Chia-Chen Chang and Chii-Wann Lin - Shenhsiung Lin, Chia-Chen Chang and Chii-Wann Lin, Biomed. Eng. Appl. Basis Commun., 2012, 24, 453.
3.Hannes Orelma, IlariFilpponen, Leena-Sisko Johansson, JanneLaine and Orlando J.; Biomacromolecules, 2011, 12, 431.
