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Microwave-millimetre wave WGM resonators for evanescent sensing of nanolitre liquid substances

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Microwave-millimetre wave WGM resonators for evanescent sensing of nanolitre liquid substances
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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/261112172 Microwave-millimetre wave WGM resonators for evanescent sensing of nanolitre liquid substances Conference Paper  · January 2009 CITATIONS 3 READS 43 5 authors , including: Some of the authors of this publication are also working on these related projects: 1.Synchronization of high temperature superconductor multijunction systems   View projectAlexey GubinO. Ya. Usikov Institute for Radio Physics and Electronics, NAS of Ukr… 47   PUBLICATIONS   305   CITATIONS   SEE PROFILE A.A. BarannikO.Ya. Usikov Institute for Radiophysics and Electronics , National Ac… 84   PUBLICATIONS   438   CITATIONS   SEE PROFILE Alexander M. KlushinInstitute for Physics of Microstructures RAS 107   PUBLICATIONS   548   CITATIONS   SEE PROFILE All content following this page was uploaded by Alexander M. Klushin on 03 April 2015.  The user has requested enhancement of the downloaded file.  Microwave-millimetre wave WGM resonators for evanescent sensing of nanolitre liquid substances Shaforost, E.N.; Klein, N.; Gubin, A.I.; Barannik, A.A.; Klushin, A.M. Microwave Conference, 2009. EuMC 2009. European Topic(s): Components, Circuits, Devices & Systems ; Fields, Waves & Electromagnetics  Publication Year: 2009 , Page(s): 45 - 48 IEEE Conference Publications  Microwave-Millimetre Wave WGM Resonators for Evanescent Sensing of Nanolitre Liquid Substances Elena N. Shaforost #1 , Norbert Klein #2 , Alexey I. Gubin *3 , Alexander A. Barannik  *4 , Alexander M. Klushin #5   #   Institut für Bio- und Nanosysteme, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany 1 o.shaforost@fz-juelich.de  2 n.klein@fz-juelich.de 5 a.klushin@fz-juelich.de *  A.Usikov Institute of Radiophysics and Electronics, National Academy of Sciences of Ukraine, 12, Acad. Proskura str., 61085 Kharkiv, Ukraine 3 gubin@ire.kharkov.ua 4 barannik@ire.kharkov.ua    Abstract   — In this paper we describe a novel approach of high sensitivity liquid analysis for volumes in the nanolitre range with challenging perspectives for practical sensor applications in chemistry, biology and medicine. Whispering-gallery modes (WGMs) in cylindrically shaped dielectric disks machined from low-loss single crystalline materials such as sapphire or quartz allow for very high quality factors. The interaction of extremely small volumes of the liquid under test with the evanescent field located in the vicinity of the dielectric disk surface at micro-to-millimetre wave frequencies was employed for the investigation of aqueous solutions with relevance to biological applications. Based on this resonator type, three different liquid sensing approaches were developed and analysed at 10, 35 and 170 GHz with emphasis on the determination of the complex dielectric permittivity of liquids of nanolitre volumes. I.I  NTRODUCTION  The investigation of liquids by electromagnetic waves in the micro- to millimeter wave bands represents a great challenge for the fundamental understanding of their dielectric and conducting properties. The structure of its molecular components, the intermolecular bonds and the electrical conductivity induced by dissolved ions expresses itself in the frequency and temperature dependence of the complex dielectric function, which contains a rich spectrum of information including various relaxation processes, orientational polarisation of the molecules, and frequency dependence of the ionic conductivity. Although broadband studies covering the largest possible frequency range represent the ultimate solution with emphasis on exploiting the full information included in the electromagnetic response of a liquid, the relative simplicity of the spectra allows to identify the main features of a spectrum  by precise measurements at a few selected frequencies. In conjunction with the high losses imposed by the dipole relaxation for water in the higher microwave frequency bands above ten gigahertz, even extremely small amounts of liquids can be analyzed, provided that resonant systems of high quality factor can be utilized as sensing elements. In the frame of this work, extremely sensitive approaches for the investigation of the dielectric properties of liquids in the frequency range from 10 to 170 GHz have been developed and analyzed, with a strong potential to be further extended to the terahertz range in the future. Cylindrically shaped dielectric resonators made from low-loss single crystals were developed, optimized and finally utilized for high precision measurements on liquids down to volumes of picolitres. Employing so-called whispering gallery modes, the dielectric resonators can be operated in semi-open metallic shielding or even without any shielding. The evanescent fields occurring near the surface of the dielectric cylinders provide an ideal  platform for the investigation of extremely small amounts of a liquid either as droplets or liquids in volumes defined by closed cavities, such as microfluidic systems. Due to the simplicity of the sensing arrangement, various applications ranging from concentration measurements on biological liquids such as glucose, monitoring of liquids in chemical,  pharmaceutical and food industry, investigation of human and tissue investigation towards label free DNA analysis can be considered. In the previous studying [1], [2] it was demonstrated that evanescent field sensing by WGM resonators is very efficient for the characterization of biochemical liquids of the smallest  possible volumes. Therefore, the resonator approach shows interesting perspectives for the development of measurement and sensor arrangements utilizing several possible schemes:  Droplet approach in various frequency bands.  A microfluidic system on a separated wafer being attached to the WGM resonator.  The open WGM resonator in conjunction with dielectric waveguide excitation for the THz range II.I  NVESTIGATION OF G LUCOSE AND A LBUMIN -W ATER D ROPLETS BY AN O PEN WGM   R  ESONATOR As first method, a whispering-gallery sapphire resonator operating at 35 gigahertz was utilized for the investigation of small droplets of glucose and lactalbumin-water solutions with volumes ranging from 100 picolitres up to about a few nanolitres. Employing a micrometer controlled microinjection system described in [2], droplets were spotted at the most sensitive positions on the surface of the sapphire disk. At the surface of the disk, an evanescent field emerges, such that the 978-2-87487-011-8 ©  2009 EuMA 29 September -  1 October 2009, Rome, Italy Proceedings of the 39th European Microwave Conference 45  droplet induces a slight change of the resonance frequency and a reduction of the inverse quality factor depending on the complex dielectric permittivity of the liquid under test. The optimum spotting position on the resonator surface corresponding to the maximum electric field was determined experimentally [2]. A sapphire disk of 14.5 mm diameter and 2.5 mm height without any shielding cavity was excited in whispering-gallery running waves HE 121  by dielectric image guides (Fig. 1 (a)). The dielectric image guides of the rectangular shape were made of Teflon and hade the following dimensions: b =2.5 mm, and 2 a =4.6 mm. The results on droplet induced changes of resonant frequency and inverse quality factor are shown in Fig. 1 (b, c). One can see that in such small volumes both solutions in concentrations down to 5% can be clearly separated from water. Both measured quantities were found to be nearly linear dependent on the droplet volume, as expected by  perturbation theory. Since the volume of the droplet is much smaller than the volume of the resonator, the effect of real and imaginary parts of permittivity could be separated. Fig. 1 Schematics of the employed resonator setup (a) composed of a sapphire disk (1), dielectric image waveguides (4), matched loads (5), microinjection pipette (3) for spotting droplets (2) on the position of maximum field. Measured droplet induced changes of (b) resonant frequency   f  liq-air    and (c) inverse quality factor 1/ Q liq -1/ Q air   as function of volume of  bidistilled water and aqueous solutions of glucose (gl) and lactalbumin (al), all measurements at T  =18°C. Theoretical modelling of the experimental results was  performed utilizing perturbation theory in conjunction with numerical field simulation of the unperturbed resonator. Slater’s perturbation formula for the case of nonmagnetic materials [3] can be presented in the following form:   1´4 200     W  E V  f  f   L  (1) In this approximation  E  0  represents an unperturbed electric field, W   is the stored resonator energy,   0  = 8.85·10 -6  As/Vm, and V   L  is the volume filled with liquid. For the change of inverse quality factor, the incremental frequency rule applies [4], such that           tan1  f  f Q  (2) with tan     =   "/    '   being the loss tangent of the liquid. In Fig. 2 the relative changes of frequency shift and change of inverse quality factor, as determined from the slopes in Fig. 1 (b, c), are presented in a form concentration dependence of relative change (in %) of dielectric permittivity with respect to the permittivity of bidistilled water, which serves as a reference liquid [5]. Fig. 2 Experimental data on (a) real '   and (b) imaginary "   parts of dielectric  permittivity as function of weight concentration (C w , %) of glucose and albumin in bidistilled water obtained from droplet measurements. (c) (b) 5   3   (a) 0 4 8 12 16 20-50-40-30-20-100 G  l  u  c  o s  e  C w  (%)      '   '   s  o   l  u   t   i  o  n   -      '   '   w  a   t    )   *   1   0   0   /      '   '   w  a   t    (   %   ) A l  b u m i  n  (b) 0 4 8 12 16 20-50-40-30-20-100 G  l  u  c  o s  e  C w  (%)      '   s  o   l  u   t   i  o  n   -      '   w  a   t    )   *   1   0   0   /      '   w  a   t    (   %   ) A l  b u m i  n  (a)   (b) 2   4   1 46  III.WGM   S APPHIRE R  ESONATOR C OMBINED WITH A P LASTIC W AFER C ONTAINING A S MALL L IQUID F ILLED C AVITY The integration of microfluidic structures with coplanar waveguide transmission lines was reported recently [6, 7]. This technique enables to measure the dielectric properties of sub-L volumes of fluids and biological samples at frequencies up to 40 GHz. As a first step towards practical microfluidic sensor applications, a measurement scheme based on whispering gallery resonances in a sapphire disk at a frequency of 11 GHz was investigated. As published recently [8], a flip-chip configuration composed of a thin quartz disk with a small hole filled with 400 nanolitres liquid and the sapphire resonator disk was already tested for highly accurate measurements of liquids. In this study, we applied a plastic rather than a quartz disk with a liquid filled cavity to be attached to the sapphire disk of 40.5 mm diameter and 10.8 mm height (see Fig. 3). The wafer was made of a new plastic material – ZEONEX cyclo olefin  polymer (COP) – which exhibits very low losses in the microwave region, extremely low water adsorption and excellent micromachining capabilities [9]. With this new assembly we achieved a quality factor of around 110 000, similar to the quartz-sapphire configuration [8]. Calibration measurements on various test liquids enable the determination of real and imaginary parts of dielectric  permittivity over a wide range of values, in accordance with analysis by perturbation theory. Similar to the droplet experiments, the liquid induced changes of resonant frequency with respect to empty cavity,   f  , and change of inverse quality factor  (1/ Q ) were measured with a vector network analyzer. The cavity was filled with the test liquid of 400 nl volume  by a Hamilton syringe-micropipette. Fig. 3 (a) Flip-chip high-Q resonator: bottom part – whispering-gallery sapphire disk, upper part plastic wafer with nanolitre cavity; (b) Photograph of the resonator assembly mounted in the semiopen copper housing. Standing whispering gallery modes HE 121  were excited in the structure by coupling loops. The liquid cavity position was optimised on the base of numerical simulation of field distribution Fig. 4 shows the calibration curves of this first prototype nanolitre liquid dielectrometer, i.e. the measured frequency and inverse Q  change as function of the corresponding real  part of permittivity (Fig. 4(a)) and loss tangent tan     =   "/    '   (Fig. 4(b)) measured by an Agilent coaxial probe technique on larger volumes. Fig. 4 Measured liquid induced changes of (a) frequency shift   f  liq-air  =  f  liq  –   f  air   as a function of real part of permittivity of liquids under test selected to cover a wide epsilon range; (b) inverse quality factor 1/ Q liq -1/ Q air   as a function of loss tangent (T = 24 °C). IV.WGM   R  ESONATOR A PPROACH AT 170   GH Z According to Eq.(1), the sensitivity for measuring of a small sample (such that perturbation theory can be applied) scales with Q/V = Q  f  3  (V= resonator volume) for a cylindrical DR of given height-to-diameter ratio and for a given mode. This means, that increasing the frequency from 35 to, say 170 GHz, already allows an increase in sensitivity  by a factor of 115, provided that the same quality factor can  be achieved. Such an increase will allow to measure dried or even frozen “liquids”, as being prepared by spotting nanolitre droplets of aqueous solutions of non-volatile substances on the resonator surface. This should allow the study of the dielectric  properties of extremely small amount of solids, for example DNA. As an inspiring example, it was shown in the literature [10, 11] that the hybridization of single DNA molecules leads to a measurable change of their electric permittivity at sub-Terahertz frequencies. This may pave a new way towards label-free DNA detection, if measurements on single pico- to nanolitre droplets of DNA-NaCl-water solutions can be  performed. plastic sapphireliquid-filledcavity (a) 0 10 20 30 40 50 600-50-100-150-200-250-300 Water     F  r  e  q  u  e  n  c  y  s   h   i   f   t   (   k   H  z   ) Real part of permittivity Ethanol 100% (a) 0.0 0.4 0.8 1.2 1.6 2.00369121518 Loss tangent    (   Q   -   1   l   i  q   -   Q   -   1  a   i  r    )  x   1   0    6 (b) (b) 47
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