Fiber Optic Plasmonic Sensors: Past, Present and Future

Transcription

1 Send Odes fo Repints to 58 The Open Optics Jounal, 2013, 7, (Suppl-1, M4) Fibe Optic Plasmonic Sensos: Past, Pesent and Futue Open Access Sachin K. Sivastava 1,* and Banshi D. Gupta 2 1 Depatment of Electo-Optic Engineeing, Ilse Katz Institute fo Nanoscale Science and Technology, Ben-Guion Univesity of the Negev, Be'e Sheva, Isael 2 Physics Depatment, Indian Institute of Technology Delhi, New Delhi , India Abstact: We eview vaious fibe optic sensos utilizing both the popagating and localized suface plasmon esonance techniques. The utilization of optical fibes in plasmon based sensing has povided seveal advantages in sensing of vaious physical, chemical and biochemical paametes. The aticle stats with a bief intoduction of the popagating suface plasmon esonance (SPR) and localized suface plasmon esonance (LSPR), pesents the eview of some of the past studies on SPR and LSPR based state-of-the-at fibe optic sensos and finally ends with the futue scope of the plasmonics based fibe optic sensos. In addition, we discuss some latest esults on the plasmon enhanced whispeing galley mode sensos, which equie tapeed optical fibes fo excitation. The pesent eview may povide the eseaches a igoous and oganized liteatue fo the undestanding of the basics, utility and tends of fibe optic plasmonic senso in a chonological ode. Keywods: Fibe optics, sensos, suface plasmons, localized suface plasmons. 1. INTRODUCTION Plasmonics has been fascinating eseaches fo last thee decades due to its ovewhelming and stuctue specific optical popeties. Both, the physics of suface plasmons as well as thei applications in vaious fields have got wide attention due to inteesting optical phenomena involved. The field of plasmonics, pimaily applied to the sensing and waveguiding applications, has now boadened to SPACERs, teatment of cances, nanoantennas, and nanofocussing, etc. [1-4]. The possibility of supe esolution, fa beyond the diffaction limit, which plasmonic stuctues posses, makes them extemely useful in ultahigh esolution and detection of single biomolecula inteactions [5, 6]. The state of the at fabication and chaacteization techniques have played cucial ole in advent of novel plasmonic and nanoplasmonic stuctues, which posses multi-esonant chaacteistics [7]. Collaboation of plasmonics with optical fibes has esulted in the miniatuized sensos with low cost, high pefomance, light weight and potability. The additional advantage due to optical fibes is the capability of emote sensing and online monitoing, which make the senso useful even in hazadous envionments. The oganization of the pesent eview is as follows: We have subdivided the aticle into two boad aeas; fist, the popagating suface plasmon esonance based fibe optic sensos and the second, localized suface plasmon esonance based sensos. Thoughout the aticle, we have used the tem suface plasmon esonance (SPR) fo popagating suface plasmon esonance. The eview stats with a bief histoy of suface plasmons (Section 2.1) and then pesents a geneal *Addess coespondence to this autho at the Depatment of Electo-Optic Engineeing, Ilse Katz Institute fo Nanoscale Science and Technology, Ben-Guion Univesity of the Negev, Be'e Sheva, Isael, Tel: , Fax: +972-(0) ; sachinchitansh@gmail.com oveview of its physics and sensing applications (Section 2.2). In section 2.3, we discuss some of ou studies on fibe optic SPR sensos, followed by influence of cetain paametes on thei pefomance (Section 2.4) and methods to enhance thei sensitivity (Section 2.5). Section 3 is devoted to localized suface plasmon esonance based sensos, in which, we again stat with a bief histoy, geneal oveview and then fibe optic LSPR sensos (Sections 3.1, 3.2, and 3.3). In section 3.4, the influence of tempeatue ove the pefomance of a fibe optic LSPR senso has been pesented. Section 3.5 discusses the enhancement of sensitivity. In section 4, we have discussed some of the ecent liteatue to point towads the new diections of futue eseach. We have then added two appendices to pesent the mathematical fomulation of suface plasmons, and localized suface plasmons to calculate the total powe at the output end of optical fibe. 2. SURFACE PLASMONS 2.1. Bief Histoy The suface plasmons have a histoy going back moe than about a hunded yeas. The fist obsevation of suface plasmons was made by Wood in 1902 [8]. It was obseved that when polaized light is incident on a mio with the diffaction gating on its suface, an anomalous patten of dak and bight bands appeas in the eflected light. Howeve in 1907, Zenneck theoetically fomulated a special suface wave solution of the Maxwell s equations and pedicted that adio fequency suface electomagnetic waves occu at the inteface of a lossy dielectic o a metal and a lossless dielectic [9]. He futhe suggested that the lossy pat of the dielectic constant of the metal was esponsible fo the electomagnetic suface wave at the inteface. In 1909, the field amplitudes of the suface waves postulated by Zenneck, wee found to vay invesely as the squae oot of the distance fom the dipole [10]. In 1941, Fano concluded that the anomalies epoted by Wood (1902) wee because of the / Bentham Open

2 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 59 Fig. (1). Schematic of a suface plasma wave at a metal-dielectic inteface. Fig. (2). Vaiation of the field of SP wave acoss the metal-dielectic inteface. excitation of suface waves on the suface of the diffaction gating [11]. Ritchie, in 1957, also pedicted the excitation of the suface plasmons on the metal suface [12]. In 1959, Tubada obseved a lage dop in the eflectivity when illuminating thin metal films on a substate, but did not link this effect to the suface plasmons [13]. In 1960, Powell and Swan obseved the excitation of suface plasmons at the metal suface using electons [14]. Soon afte, in 1960 only, it was concluded by Sten and Feell that the electomagnetic waves at the metallic suface possessed electomagnetic adiation intemingled with suface plasmons [15]. In 1968, Otto explained Tubada s esults and demonstated that the dop in the eflectivity in attenuated total intenal eflection is due to the excitation of the suface plasmons [16]. Howeve the Otto configuation was not vey much suitable fo the pactical point of view due to the finite gap between the metal laye and the pism base. In 1971, Ketschmann modified the Otto configuation by emoving the space between the metal laye and the pism [17]. This scheme has been found to be bette than pevious configuations, fom application point of view. Since then the Ketschmann configuation has been utilized in vaious sensing applications Geneal Oveview Suface plasmons ae the quanta of chage density oscillations at a metal dielectic inteface. These oscillations get excited when the enegy and wave-vecto of incident light at the inteface becomes equal to that of suface plasmon modes suppoted by the inteface. As a esult of these oscillations, unde the action of the apidly changing electomagnetic field, a wave, called suface plasma wave, gets geneated, which tavels along the inteface and its field amplitude decays exponentially in both the metal and the dielectic medium (Fig. (1) and Fig. (2)). It can be obseved in Fig. (2) that the field amplitude decay in metal (cuve (b)) is faste than that in the dielectic medium (cuve (a)). This can be

3 60 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (3). Dispesion cuves fo the suface plasmon wave and the diect light. Fig. (4). A Schematic of Ketschmann Configuation. attibuted to the imaginay pat of the metal dielectic function. Also, these waves ae tansvese magnetically (TM) polaized and get excited by only TM polaized light. In geneal, suface plasmons cannot be excited at a metal-dielectic medium inteface by diectly shining it by a beam of light, because the wave-vecto of the light in the dielectic medium is always smalle than that of the suface plasmons at the inteface. It can be bette undestood by plotting the dispesion cuves fo a light wave and a suface plasmon wave. The dispesion cuves fo both the suface plasmons at ai-gold inteface and light in ai have been plotted in Fig. (3). The staight line (a) in Fig. (3) coesponds to light line following the dispesion elation! = ck, the cuve (b 1 ) coesponds to suface plasmons bound to the inteface, while (b 2 ) coesponds to adiative plasmons. It can be obseved that fo a light wave of fequency! o, the wave vecto of suface plasmon is geate than that of light. Theefoe light diectly incident fom the dielectic medium at the metal dielectic inteface cannot excite suface plasmons [18]. Fo fequencies geate than the plasma fequency! pb (egion (b 2 )), the plasmonic modes ae called adiative and the metal becomes tanspaent. Thee ae special techniques fo the excitation of suface plasmons at a metal dielectic inteface, in which the wavevecto of the incident light is inceased to match with suface plasmons. Two most geneal techniques ae named as Otto and Ketschmann configuations. Some peiodically stuctued sufaces have also been used fo inceasing the wavevecto of the incident light. In such a case, howeve, the dielectic medium needs to be quite tanspaent, so that light can be collected in eflection mode. Fig. (4) shows the schematic of a typical Ketschmann configuation. In a Ketschmann configuation, the base of a high efactive index pism is coated with a vey thin laye of nobel metal,

4 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 61 Fig. (5). SPR cuves fo a Ketschmann configuation, with pism made of S10 glass, gold film of 50 nm thickness. Fig. (6). SPR dispesion cuves fo a Ketschmann like configuation. which is futhe suounded by the dielectic medium. The light eflected off the othe face is fed to a detecto. The evanescent field at the pism metal inteface excites the suface plasmons at the metal-dielectic inteface, when the fequency and wave-vecto of the incident light becomes equal to that of suface plasmon mode. This is temed as sufce plasmon esonance (SPR). As a esult of the esonance, a dip is obtained in the eflected powe, coesponding to a paticula angle at which the wave vectos match. Fig. (5) shows the vaiation of eflectivity with angle fo dielectic media of two diffeent efactive indices. It is obseved that the esonace position changes with a change in the efactive index of the dielectic medium. With an incease in the efactive index of the suounding medium, the esonance angle inceases. This popety of suface plasmons is utilized in sensing applications. Fig. (6) shows the dispesion cuves of suface plasmons and incident light fo a Ketschmann like configuation. It is obseved that fo a cetain angle of incidence at the pismmetal inteface, fo a monochomatic light, the wave-vectos of both the suface plasmons at the metal-dielectic inteface and incident light match. Howeve, light incident though the pism cannot excite suface plasmons at the metal-pism inteface, as the wave vecto of the suface plasmon mode at this inteface is geate than that of light in the pism. Mathematically, it can be undestood as follows: The wave vecto of suface plasmon (k sp ) is given by the following elation "! k sp = k m! s % o $ #! m +! ' s & 1/2 whee, k o is the wave vecto of light in fee space and m and! s ae the dielectic constants of the metal and the dielectic medium, espectively. The wave vecto of the evanescent wave is given by following elation k ev = k p sin! = " c (1) # p sin! (2)!

5 62 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (7). Schematic of a fibe optic SPR senso. whee k p is the wave vecto of the incident light in pism with dielectic constant ε p, ω is the angula fequency and c is the speed of light in fee space. The complete tansfe of enegy takes place when the wave vecto of evanescent wave and the suface plasmon wave become equal, o the following elation is satisfied! c " p sin# es =! $ " m " s ' c & % " m + " ) s ( 1/ Fibe Optic SPR Sensos Among the fist few epots on fibe optic SPR sensos, the wok epoted by Villuendas and Palayo got attention of eseaches [19]. Thei wok consisted of expeimental esults on aqueous sucose solutions fo sensitivity and dynamic ange evaluation. Soon afte, a fou laye fibe optic SPR senso was epoted with enhanced sensitivity and dynamic ange [20]. Aound the same time, a single mode optical fibe SPR senso with SPR excitation on the tip was epoted [21]. Among these epots, the one epoted by Jogenson and Yee got the most attention, due to a complete theoetical and expeimental study of fibe optic SPR sensos, with an application in chemical sensing [22]. Jogenson and Yee eplaced the pism of the Ketschmann configuation by the coe of an optical fibe and excited suface plasmons by the evanescent field at the coe-cladding inteface. Thei senso was well accepted because SPR sensos utilizing optical fibes have been found to povide multiple advantages ove pism based sensos. These sensos ae simple in design, miniatuized, low cost, highly sensitive and accuate. Use of optical fibe futhe adds the advantage of online monitoing, emote sensing and feedom fom electic shocks and electomagnetic intefeences. The compatibility of gold with vaious biological species and functional goups makes these sensos useful fo biosensing applications. Since then, vaious fibe optic sensos have been epoted [23]. A numbe of attempts have been made to enhance the sensitivity of these sensos by vaious pobe designs and additional layes [24, 25]. These sensos have been employed in sensing of diffeent chemicals and biochemicals and envionmental and health monitoing [26-28]. Bimetallic and alloy films have also been used to enhance the pefomance [29, 30]. Some theoetical epots have pesented the collaboation of plasmonic stuctues with fibe Bagg gatings (FBGs) and long peiod gatings (LPGs) [31]. Howeve, the SPR based FBG sensos have not yet been ealized pobably due to expeimental difficulties and still optimization of cetain paametes is equied befoe fabication. The fibe optic (3) intefeometic sensos utilizing SPR have also been studied in Faby-Peot configuation [32]. In a fibe optic SPR senso, geneally, the cladding of a small potion of the optical fibe is emoved fom the middle and coated with a thin laye of metal. The dielectic medium to be sensed futhe suounds the metal coated egion as shown in Fig. (7). Polychomatic light launched fom one end of the optical fibe gets guided into it by the phenomenon of total intenal eflection at the coe-cladding inteface. The evanescent field of the light guided into the optical fibe excites suface plasmons at the metal coated egion. As a esult of SPR, the powe tansmitted off the output end of the fibe shows a dip in the tansmission spectum. This happens due to the absobance of the light of a paticula wavelength by suface plasmons. When the efactive index of the dielectic medium aound the sensing egion is changed, the position of the dip in the tansmission spectum gets changed. A schematic of an expeimental setup to chaacteize a fibe optic SPR senso is shown in Fig. (8). Befoe launching of light in the optical fibe, the senso pobe is fixed in a glass flow cell having inlet and outlet facilities fo liquid sensing samples. Next we pesent some of the studies on fibe optic SPR sensos. (a) Detection of Low Wate Contents in Ethanol Ethanol is seen as a pospective fuel which may eplace the dependence on conventional fossil fuels. It has been given moe impotance due to limited amount of fossil fuels and pollution concens. Howeve, ethanol must be at least 93% pue to be used as a fuel [34]. Apat fom that, ample amounts of ethanol ae equied in medical industy, biological sciences and oganic chemisty, whee it is needed in puest foms. The ethanol is mostly manufactued by its extaction fom molasses and contains wate as a miscible impuity. Hence, it is vey impotant to detect the amount of wate in ethanol and emove it. Wate in ethanol mixtues of vaying wate pecentages in ethanol wee pepaed and coesponding SPR specta wee ecoded by using the expeimental setup shown in Fig. (8). Fig. (9) shows the ecoded SPR specta fo diffeent pecentages of wate in ethanol [33]. It is obseved that with the inceasing wate pecentage in ethanol, the SPR specta fist show a ed shift and then afte a cetain wate pecentage, they shift towad blue wavelength egion. This kind of behavio is totally confusing as the efactive index vaiation must vay unifomly with the incease of wate pecentage. The eason behind such a behavio is attibuted to the fact that at small wate pecentages

6 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 63 Fig. (8). Schematic of the expeimental setup used fo the study of SPR specta. THL: tungsten-halogen lamp, MO: micoscope objective, 3D-TS: thee dimensional tanslation stage, OF: optical fibe, FC: flow cell, SP: spectomete, CC: connecting cable, COMP: compute. (Repinted with pemission fom Elsevie [33]). Fig. (9). SPR specta fo diffeent wate contents in ethanol-wate mixtue (Repinted with pemission fom Elsevie [33]).

7 64 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (10). Vaiation of esonance wavelength with wate content in ethanol-wate mixtue (Repinted with pemission fom Elsevie [33]). Fig. (11). Vaiation of esonance wavelength with wate content in ethanol-wate mixtue fo the ange 0 to 10% (Repinted with pemission fom Elsevie [33]). in ethanol, small clustes of ethanol-wate ae fomed due to local hydogen bonding [35]. As the wate pecentage inceases, the density of these clustes inceases and hence the efactive index ises a bit. Afte a cetain wate pecentage, futhe addition of wate leads to the unifom hydogen binding thoughout the mixtue, which esults in disappeaance of the clustes, giving ise to decease in the efactive index. The shift in esonance wavelength with wate pecentage is plotted in Fig. (10) [33]. It can be seen that eithe the ising phase o the falling phase of the chaacteistic cuve can be used fo sensing, as fo a cetain ange, the same esonance wavelength coesponds to two wate pecentages. Since, ethanol is needed in puest foms fo its vaious applications; the egion of inteest is the ising phase of the chaacteistic cuve. The vaiation of esonance wavelength with wate pecentage up to 10% wate in ethanol is shown in Fig. (11) [33]. It is obseved that the esponse of the senso in this ange of inteest

8 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 65 Fig. (12). Vaiation of esonance wavelength with glucose concentation (Repinted with pemission fom SPIE [36]) Fig. (13). Schematic of stepwise pepaation of AAB/4-ATP/Au modified fibe suface fo binding of LDL. (Repinted with pemission fom IEEE [37]). is linea. The sensitivity calculated by the slope of the plot was nm/ wate % in ethanol [33]. (b) Glucose and Low Density Lipopotein Biosensos Fibe optic SPR sensos have igoously been used fo biosensing applications [26-28]. To fabicate a biosenso, eceptos of desied molecule ae geneally immobilized ove the metal film and suitable analytes ae then flown ove the immobilized potion. A glucose senso was fabicated by immobilizing a self assembled monolaye (SAM) of glucose oxidase ove the gold laye [36]. Fig. (12) shows the vaiation of SPR wavelength with glucose concentation. It is obseved that the SPR wavelength inceases with the incease in the glucose concentation. The sensitivity of the senso was found to be nm/ (mg/dl). Simila effots wee made to fabicate a SPR based fibe optic low density lipopotein (LDL) senso [37]. The detection of LDL is a cucial issue, as it is bad cholesteol and is esponsible fo the heat diseases. The senso was fabicated by immobilizing anti-apolipopotein B (AAB) ove gold. Fig. (13) shows the stepwise fabication of the senso by SAM fomation ove gold [37]. The gold coated optical fibe was fist functionalized by 4- Aminothiophenol (4-ATP), which was futhe immobilized by antiapolipopotein B (AAB). Befoe the

9 66 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (14). Vaiation of esonance wavelength with concentation of LDL fo fibe optic SPR pobes with and without SAM layes. (Repinted with pemission fom IEEE [37]). immobilization of AAB ove 4-ATP/Au suface, AAB was fist coss-linked by EDC (N-ethyl-N-(3-dimethylaminopopyl cabodimide)-nhs (N-hydoxysuccinimide) chemisty to incease the coupling efficiency by folds [38]. The conjugation of AAB with EDC-NHS allowed diect immobilization of AAB on 4-ATP/Au suface. The pepaed AAB/4-ATP/Au optical fibe pobe was then incubated in BSA (2mg/ml) solution fo one hou at oom tempeatue to block those 4-ATP sites which emain unbound by AAB and ae unspecific fo the binding of LDL. This step helps in emakably educing the binding of any foeign element which may give false esponse and makes the senso specific to low density lipopotein (LDL) only. The esponse of the senso with and without the SAM layes is shown in Fig. (14) [37]. It is obseved, that the esponse of the senso is linea in the ange of inteest. The expeimental cuves fo the senso without SAM laye wee obtained by a contol expeiment pefomed to check the ole of SAM laye in the sensing. It was obseved that thee is no change in esonance wavelength with change in LDL concentation fo a senso without SAM laye. The esponse of the senso in the ange of opeation was found to be linea. The sensitivity of the senso, calculated fom the esponse cuve, was found to be nm pe mg/dl concentation of LDL Influence of Paametes othe than Refactive Index Fom the above, it is clea that a change in efactive index of the sensing medium leads to a change in the suface plasmon esonance wavelength. This gives ise to a plain cuiosity that is it only the efactive index of the dielectic medium which changes the plasmon esonance o some othe paametes can affect the plasmon esonance condition. Lioubimov et al studied the effect of vaying electic potential on the pefomance of a SPR senso [39]. The effect of tempeatue on the pefomance of a fibe optic SPR senso was studied and the effects of tempeatue on sensitivity and signal to noise atio (SNR) fo two plasmonic metals silve and gold wee discussed [40]. The influence of vaious dopants in the optical fibe coe on the pefomance of fibe optic SPR senso was studied [41]. It was concluded that the sensitivity of the senso inceases with the incease in doping concentation of B 2 O 3, while the SNR emains unaffected. Futhe, the influence of skew ays on the pefomance of a fibe optic SPR senso was studied and it was concluded that skew ays degade the pefomance [42]. The influence of ions on the pefomance of a fibe optic SPR senso has also been studied [43]. Fig. (15) shows the vaiation of suface plasmon esonance wavelength with efactive index fo fou ionic and two non-ionic aqueous solutions. It was found that, fo the same efactive index of the sensing medium, the shift in esonance wavelength is geate fo ionic media than that fo non-ionic ones. Also, the shift in esonance wavelength was found to be diffeent fo diffeent ionic media. Thus, the ions ae found to affect the SP esonance condition. To confim the ole of the ions, the vaiation of ion concentation with efactive index was studied. It was found that, fo the same value of the ion concentation, the efactive index of Na 2 SO 4 and MgCl 2 is geate than NaCl and KCl as shown in Fig. (16) [43]. Thus, it was concluded that the shift in esonance wavelength inceases with an incease in the ion concentation in the sample solution Enhancement of Sensitivity Vaious effots have been made fo the enhancement of sensitivity of fibe optic SPR sensos. Fist step towads the enhancement of sensitivity was the tapeing of optical fibe pobes [44, 45]. Single and dual tapeed optical fibe SPR sensos have also been epoted [46, 47]. The effect of vaious tape pofiles on the pefomance of an optical fibe SPR senso was studied fo the optimization of the pefomance [48]. It was concluded that the sensitivity of the senso is maximum when the tape pofile is exponential linea. Futhe, a U- shaped fibe optic SPR senso was theoetically studied fo the enhancement of sensitivity [49]. In both the

10 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 67 Fig. (15). Vaiation of esonance wavelength (λ es ) with the efactive index of sucose, uea, Na 2 SO 4, MgCl 2, NaCl and KCl solutions (Repinted with pemission fom Elsevie [43]). Fig. (16). Vaiation of ion concentation with the efactive index of sucose, uea, Na 2 SO 4, MgCl 2, NaCl and KCl solutions. (Repinted with pemission fom Elsevie [43]). cases, the tapeing and U-shaped pobes, an enhancement in the stength of coupling of the evanescent field with suface plasmons takes place, which is esponsible fo the incease in the sensitivity. Howeve, these sensos futhe suffe the disadvantage of fagility and educed detection accuacy, as the SPR cuves boaden with tapeing. To povide obustness to the tapeed fibe pobes, multi-tapeed fibe optic SPR sensos wee pepaed [50]. In such pobes, the tape atio (atio of the untapeed diamete to the tape waist) was kept close to unity and multiple tapes wee witten in the same sensing length, shown in Fig. (17). The distance between two tapeed egions was temed as tape peiod (Λ). Fig. (18) shows the vaiation of the esonance wavelength with efactive index fo diffeent tape peiods [50]. It was obseved that with an incease in the numbe of tapes the esonance wavelength showed a ed shift, while the total shift in esonance wavelength was also inceased. The ed shift in esonance wavelength was attibuted to the fact that tapeing leads to an incease in the penetation depth. Since the penetation depth is popotional to the wavelength, the coupling of light at highe wavelengths becomes stonge, in tun ed shifting the esonance wavelength. Futhe, the tapeing inceases the numbe of eflections within the sensing length, which inceases the sensitivity, but also deceases the detection accuacy. Few eseaches epoted fibe optic plasmonic sensos with a vey thin laye of silicon ove metal film [51]. These sensos wok on the pinciple of guided wave plasmonics and possess bette sensitivity than usual fibe optic plasmonic sensos. Fibe optic sensos utilizing guided wave

11 68 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (17). Micoscopic view of the tapeed optical fibe coe (Repinted with pemission fom IEEE [50]). Fig. (18). Vaiation of λ es with efactive index the pobes with diffeent Λ (Repinted with pemission fom IEEE [50]). plasmonics have also been fabicated fo the detection of uea [52]. 3. LOCALIZED SURFACE PLASMONS 3.1. Bief Histoy The field of localized suface plasmons is much olde than that of popagating ones. The gold nanopaticles ae known fom ancient times and wee oiginally used in staining glasses. The soluble gold appeaed pobably in the 4 th o 5 th centuy B.C. in China and Egypt [53]. The Lycugus Cup in the Bitish museum, made by the Romans in the fouth centuy AD, appeas geen in the eflected light (in daylight). Howeve, when viewed in light tansmitted though the glass, it appeas ed. The ed colo is due to small paticles of metallic gold [54]. Ancient stained glass makes knew that by putting vey tiny amounts of gold and silve in glass they could poduce ed and yellow colos. By placing small amounts of tiny nanopaticles in some paticula fashion, the physical popeties of a mateial can be changed. In the fifteenth and sixteenth centuies, the pottes stated using glazes containing coppe and silve nanopaticles. In the sixteenth centuy (about 1548), Cipiano Piccolpasso povided the ecipe fo the nanopaticulate glazes in his book Li te libi dell ate del vasaiol. He used a mixtue of coppe and silve salts with vinega, oche, and clay and applied these mixtues to the suface of aleady glazed pottey [55]. The eputation of gold sols until the middle ages was to exploe its fabulous cuative powes fo vaious diseases, such as heat and veneeal poblems, epilepsy, dysentey and tumos and fo the diagnosis of syphilis. The colloidal gold sols wee called auum potabile (dinkable gold) by the alchemists of the Middle Ages. The fist book on colloidal gold was Panacea Auea-Auo Potabile, witten by Fancisci Antonii in This book povides consideable infomation about the pepaation of gold sols and thei uses in medical applications, including some pactical cases [56]. In 1676, a Geman chemist, Johann Kunckels, published a book named Nuetaliche Obsevationes ode Anmekungen von Auo und Agento potabili which descibed the then ecent obsevations on gold and silve sols and thei applications [57]. In 1685, Andeas Cassius published a teatise De

12 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 69 Fig. (19). Schematic of a spheical metal nanopaticle unde the action of a apidly vaying field. Auo, which povided the ecipe fo making the colouing pigment o stain by gold [58]. Since then the colou has been called Puple of Cassius. A complete teatise on colloidal gold was witten by Hans Heinich Helcha in the beginning of the eighteenth centuy [59]. In this teatise, he stated that the use of boiled stach in its dinkable gold pepaation noticeably enhanced its stability. In 1794, Fulhame wote a book which epoted that silk could be dyed by colloidal gold [60]. In 1818, Jeemias Bejamin Richtes explained the diffeences in colos shown by gold sols pepaed by diffeent methods [61]. He pointed out that the pink o puple solutions contain gold in the finest degee of subdivision, wheeas the yellow solutions esult due to the fomation of aggegates of the paticles. Faadey, in 1857, epoted the fomation of deep ed colloidal gold solutions by the eduction of an aqueous solution of tetachlooauate using phosphous in CS 2 [62]. He investigated the optical popeties of thin films of died colloidal solutions and obseved evesible colo changes upon mechanical compession. This was pobably the fist use of metal nanopaticles fo any sensing application (pessue). Aound 1897, almost 250 yeas afte the discovey of Puple of Cassius, Richad Zsigmondy showed that it consisted of colloidal gold and stannic acid. He was awaded the Nobel Pize in Chemisty in In the 20th centuy, vaious methods fo the pepaation of gold colloids wee epoted and eviewed. Tukevich et al. (1951) epoted the pepaation of colloidal gold paticles by eduction of aqueous solution of tetachlooauic acid by sodium citate [63]. The method was modified by many eseach goups to obtain paticles of unifom size and shape. The theoetical analysis of small nanostuctues stated in 1908, when Gustav Mie developed a theoy of scatteing and absoption exhibited by small colloidal paticles of gold suspended in wate [64]. Aound the same time, Pete Debye solved the poblem of adiation pessue exeted on small paticles in fee space, one of the fist applications of Mie theoy to the astophysical poblem. Since then vaious attempts have been made by Keibig and othes to study the optical popeties of small metal nanostuctues due to the changes in thei electonic popeties at such a small level [65] Geneal Oveview Localized suface plasmons ae the quanta of chage density oscillations in metal nanopaticles. They get excited when the fequency of the incident light becomes equal to that of oscillation fequency of conduction electons of the metal nanopaticle. Fig. (19) shows the schematic of the action of a vaying electic field on a spheical metal nanopaticle. It can be seen in Fig. (19) that the conduction electon cloud of the nanopaticle gets shifted in a diection opposite to the diection of the electic field vecto of the incident field, leaving behind the positive backgound. Thus, the oveall positive and negative chage centes of the nanopaticles get displaced depending upon the stength of the electic field and thus behave like a dipole. Unde the action of apid vaiation of incident electic field (~ /sec), this dipole stats oscillating. At a cetain incident fequency, the polaizability of the nanopaticle shows enhanced esonant oscillations. This esults in the absobance of incident light and is called localized suface plasmon (LSP) esonance. The plasmon esonances of metal nanopaticles ae chaacteized by thei extinction coss-section, which is the sum of its scatteing and absoption coss-sections. The extinction cosssection is highly dependent on the shape, size, mateial and envionment of the metal nanopaticle [66]. The dependence of the LSP esonance wavelength on the efactive index of the nanopaticle is geneally utilized fo the sensing applications. Fig. (20) shows the LSPR specta fo a spheical gold nanopaticle of 5 nm adius suounded by thee diffeent dielectic media. As the efactive index of the medium suounding (! s ) the nanopaticle inceases, the esonance wavelength gets ed shifted. By measuing the change in the esonance wavelength, one can pedict the

13 70 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (20). Simulated LSPR specta fo diffeent dielectic constants of the suounding medium. Fig. (21). Schematic of a fibe optic LSPR senso. change in efactive index value. Metal nanopaticles of vaious shapes and sizes have been utilized by many eseaches fo sensing applications [66] LSPR Based Fibe Optic Refactive Index Senso Fibe optic LSPR sensos have got attention due to thei compact size and low cost. The most geneal scheme of such a senso is shown in Fig. (21). In such a senso, a small potion of the cladding is emoved fom the middle of the optical fibe and a monolaye of the metal nanopaticle is coated ove the unclad potion. The sensing medium futhe suounds the nanopaticle coated egion. The evanescent field of the light guided inside the coe of the optical fibe excites the plasmon esonance of the metal nanopaticles. As a esult, a small potion of the guided spectal ange gets absobed by the nanopaticle laye and a peak is obseved in the absoption spectum of tansmitted optical powe. Fist expeiment on such senso was caied out by Chau et al. fo biosensing applications [67]. Lin et al epoted LSPR based fibe optic eflection pobe fo chemical sensing at high pessue conditions [68]. Late they pesented a LSPR based Cd senso [69]. Aound the same time, Sai et al studied LSPR sensos fo immunoglobulin- anti immunoglobulin (IgG-anti IgG) inteactions [70]. We now pesent below some studies on LSPR based fibe optic sensos. (a) Refactive Index Senso Simulations wee caied out to study a fibe optic LSPR based efactive index senso [71]. The esponse of the senso was studied fo vaious light launching conditions and light souces such as collimated monochomatic and polychomatic souce and diffuse souce like LEDs. Figs. (22) and (23) show the vaiation of effective evanescent coefficient pe nanopaticle with dielectic constant fo a diffuse souce such as light emitting diode (LED) having Gaussian powe pofile [71]. The senso esponse in this case is studied fo intensity modulation scheme of opeation. It was obseved that with an incease in the dielectic constant of the suounding medium, the effective evanescent absobance pe nanopaticle inceases, eaches to a maximum at cetain dielectic constant value and then again stats deceasing. This can be undestood as follows: A change in the dielectic constant of the sensing medium changes the position of the peak of the LSPR spectum of the nanopaticle. This leads to the change in ovelap of the LSPR spectum with the pofile of the light souce. Since, the LSPR spectum shows a ed shift with an incease in the sensing medium dielectic constant, the ovelap of the LSPR spectum fist inceases with incease in dielectic constant, eaches to the maximum ovelap, and then stats deceasing with futhe incease in the dielectic constant, as the LSPR spectum now moves away fom the souce bandwidth egion. (b) Tempeatue Senso A LSPR based fibe optic tempeatue senso having gold nanopaticles in the coe of the optical fibe was ealized by Dhawan and Muth [72]. The sensitivity of the senso was, howeve, low. Şayan and Ӧzdemi [73] used TiO 2 in a pism based SPR senso while Shama and Gupta [74] used it in fibe optic SPR senso fo tempeatue sensing. The simulations wee pefomed to use LSPR fo fibe optic tempeatue sensing [75]. Fig. (24) shows the schematic design of the simulated fibe optic LSPR pobe. The pobe consists of spheical gold nanopaticles unifomly dispesed in a sensing

14 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 71 Fig. (22). Vaiation of evanescent absobance with dielectic constant of the sensing medium fo two diffeent diffuse souces. (Repinted with pemission fom OSA [71]). Fig. (23). Vaiation of effective evanescent absobance with dielectic constant of the sensing medium fo diffeent souce bandwidths (Repinted with pemission fom OSA [71]). Fig. (24). Schematic of the poposed fibe optic tempeatue pobe utilizing metal nanopaticles. (Repinted with pemission fom OSA [75]).

15 72 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (25). Vaiation of diffeence in peak esonance wavelengths (λ max - λ o ) with tempeatue fo SiO 2, TiO 2, ZnGeP 2 and CdGeP 2 as suounding media. λ o is the coesponding peak esonance wavelength at 27 o C. (Repinted with pemission fom OSA [75]). medium, which is coated on the unclad potion of the optical fibe. The efactive index of the sensing medium was kept geate than that of coe and cladding. In such case, the light guided in the optical fibe gets efacted at the coe-sensing medium inteface, excites localized suface plasmons, eaches the sensing medium-ai inteface, fom whee it gets total intenally eflected and then comes back to the coe. The senso woks on spectal inteogation method. The simulations wee caied out fo fou diffeent dielectic media (SiO 2, TiO 2, ZnGeP 2 and CdGeP 2 ) as sensing medium. SiO 2 and TiO 2 wee consideed as sensing media because these have aleady been used in vaious tempeatue sensing applications [72-74], while the tempeatue sensing popeties of ZnGeP 2 and CdGeP 2 wee exploed fo the fist time in the study. The themo-optic coefficients of these mateials ae high. The esonance wavelengths calculated fom the simulated LSPR specta fo the fou sensing media wee plotted with tempeatue and ae shown in Fig. (25). It was obseved that the change in esonance wavelength with tempeatue is the maximum in consideed mateials. The sensitivity of the senso, calculated fom the slope of cuve shown in Fig. (25), is tabulated in Table 1. It was found that the sensitivity of the senso with CdGeP 2 as sensing medium is 10 times geate than SiO 2 and TiO 2, which ae conventional mateials used fo tempeatue sensing. Though TiO 2 has a high themo-optic coefficient, it being negative, cancels the effect of tempeatue on the shift in LSPR wavelength. (c) Hydogen Gas Senso The plasmon esonances of palladium (Pd) nanopaticles wee used fo fibe optic hydogen gas (H 2 ) senso by Pabhaka et al. [76]. Palladium is a good absobe of H 2 and it is widely used fo H 2 sensing applications. But, as discussed above in ealie cases, it has always been used as a sensing medium. Howeve, the senso fabicated by Pabhaka and co-wokes woks on the pinciple of change in the dielectic constant of the metal nanopaticle by absoption of H 2. Table 1. Sensitivity of the Senso fo Diffeent Dielectic Mateials. (Repinted with Pemission fom OSA [75]) Sensing Mateial Sensitivity (µm/ o C) SiO TiO ZnGeP CdGeP Influence of Tempeatue It has aleady been established that the pefomance of a SPR senso gets affected by any change in the ambient tempeatue [77]. Howeve, it is highly demanded that the pefomance of a senso does not change with any fluctuation in tempeatue. So, the pefomance of a fibe optic LSPR senso was studied, both theoetically and expeimentally, unde the changing ambient tempeatue conditions [78]. Futhe, the plasma and collision fequencies of metal nanopaticle ae tempeatue dependent. This leads to a change in the eal and imaginay pats of the dielectic constant of the metal nanopaticle with a change in ambient tempeatue. Fig. (26) shows the wavelength dependent vaiation of (a) eal (ε 1 ) and (b) imaginay (ε 2 ) pats of the dielectic constant of a spheical metal nanopaticle of 5 nm adius at diffeent tempeatues. It is obseved that both ε 1 and ε 2 incease with an incease in tempeatue. Since the eal pat of the dielectic constant inceases with tempeatue, a ed shift in the LSPR spectum will occu with incease in tempeatue.

16 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 73 Fig. (26). Vaiation of the dielectic constant of the spheical metal nanopaticle of adius 5 nm with wavelength at diffeent tempeatues. (a) eal pat, and (b) imaginay pat. (Repinted with pemission fom OSA [75]). Fig. (27). Expeimental and simulated esults fo the vaiation of esonance wavelength with the tempeatue of the wate as the sensing medium. ((Repinted with pemission fom IFSA [78])). The tempeatue dependence of the dielectic functions of optical fibe coe, metal nanopaticle and sensing medium was consideed fo simulations. Wate was used as sensing medium because it is the mostly used solvent and is a medium fo almost all the biological entities. The chaacteistics of the senso epoted have been plotted in Fig. (27) and (28), both in (a) wavelength and (b) intensity modulation scheme of opeation. The numeical esults have been calculated by consideing only single nanopaticle. Fom both the plots, it was concluded that in spectal inteogation scheme of opeation, the pefomance of the senso emains nealy unaffected, as only 3 nm shift in esonance wavelength was obseved fo a tempeatue change of 7 C to 100 C; while in intensity modulation scheme, the pefomance of the senso got affected and a calibation facto depending upon tempeatue must be included in the chaacteistic cuve Enhancement of Pefomance The sensitivity of an optical fibe evanescent wave absoption senso has been found to incease eithe by tapeing o by making a U shaped pobe [46, 79]. In both the cases, the evanescent field of the guided light extends moe in the sensing egion, leading to inceased absobance, and hence sensitivity. In the case of fibe optic LSPR sensos, it has been epoted in liteatue that the intensity modulation mode of opeation is moe sensitive than the spectal inteogation scheme. Hence, by making U-shaped pobes, the absobance of light by metal nanopaticles can be enhanced. The LSPR based U-shaped fibe optic senso was fabicated fo studying the inteaction between IgG- anti IgG [70]. The LSPR based U-shaped fibe optic senso povides the futhe advantage of point sensing, which leads to miniatuization of sensing sample volume. A U-shaped fibe optic LSPR senso

17 74 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (28). Vaiation of diffeence in absobance (A t A 7 ) at any tempeatue with efeence to 7 C with tempeatue (Repinted with pemission fom IFSA [78]). Fig. (29). Micoscopic image of the pepaed U-shaped optical fibe pobe (Repinted with pemission fom Spinge [80]). was fabicated fo the detection of blood glucose [80]. Fig. (29) shows the micoscopic view of a U-bent optical fibe. Such a pobe was fist immobilized with gold nanopaticles and then by glucose oxidase by chemical binding. The schematic of the expeimental setup used fo the chaacteization of the senso is shown in Fig. (30). Light launched fom a polychomatic souce was fed into the optical fibe, in which the evanescent field excites LSPR and then was collected off the output end by a spectomete intefaced with a compute. Fig. (31) shows the LSPR specta ecoded fo diffeent concentations of aqueous solutions of glucose vaying fom 0 mg/dl to 250 mg/dl. This ange of glucose concentation was chosen to mimic the human blood glucose concentations. It was obseved that as the glucose concentation inceases, the LSPR absobance deceases. Howeve, a negligible shift in the esonance wavelength with incease in the glucose concentation was obseved. The eason behind the negligible shift in esonance wavelength was epoted to be due to the negligible change in the efactive index of the sample solutions in the consideed concentation ange. The change in absobance occus due a small change in the local efactive index of glucose oxidase because of the following eaction Glucose + O 2 Glucose Oxidase!!!!!" GluconicAcid + H 2 O 2 As the concentation of glucose in the vicinity of the sensing egion inceases, the change in the local efactive index inceases which leads to lage change in the absobance. The decease in absobance with glucose concentation can easily be obseved in the inset. The esponse of the senso changes with change in the bending adius of the U- shaped pobe. This happens due to the change in penetation

18 Fibe Optic Plasmonic Sensos: Past, Pesent and Futue The Open Optics Jounal, 2013 Volume 7 75 Fig. (30). Schematic expeimaental set-up fo the chaacteization of U-shape LSPR (Repinted with pemission fom Spinge [80]). Fig. (31). Absobance specta of U- shaped LSPR pobe of bending adius mm fo diffeent concentations of glucose. (Repinted with pemission fom Spinge [80]). depth in the case of diffeent bending adii. The sensos with diffeent bending adii wee fabicated fo the optimization of the pefomance of the senso. The LSPR specta wee ecoded fo all the sensos and the sensitivity was evaluated fom them. The vaiation of sensitivity with the bending adius is shown in Fig. (32) fo the fixed glucose concentation of 100 mg/dl. It was obseved that, fo a fixed glucose concentation, the sensitivity of the senso inceases with decease in the bending adius, eaches a maximum value fo cetain bending and then again stats deceasing with futhe bending. The eason behind such a vaiation can be undestood as follows: As we stat bending an optical fibe, the evanescent field stats penetating moe to the sensing egion, which, in tun, leads to an incease in the stength of coupling between the light and localized suface plasmons. This happens up to a cetain bending adius. Fo futhe incease in the bending (i.e. decease in the bending adius), the light stats leaking in to the oute medium, which is esponsible fo the decease in the sensitivity. The sensitivity of a U-shaped fibe optic LSPR senso was obtained to be maximum aound 1mm bending adius. 4. THE FUTURE Optical fibe SPR sensos fo multianalyte and multichannel sensing have been ealized ecently. A igoous theoetical and expeimental study of such senso was pefomed ecently by Vema et al. [81]. In such a senso, dif-

19 76 The Open Optics Jounal, 2013, Volume 7 Sivastava and Gupta Fig. (32). Vaiation of sensitivity with bending adius at the glucose concentation 100 mg/dl (Repinted with pemission fom Spinge [80]). feent egions of an optical fibe with diffeent plasmonic mateials can be utilized fo simultaneous sensing of multiple analytes. The senso uses the diffeent spectal egions of the visible light spectum. It is anticipated that, in coming futue, lot of applications of multichannel sensing may come to ealization of sensing of multiple analytes. The simultaneous collaboation of SPR and LSPR in the same optical fibe sensing pobe is also foeseen. The plasmonic esonances have now eached to the detection limit of single molecules [6]. People have now stated fabicating novel plasmonic stuctues fo sensing applications [82]. These sensing stuctues can be collaboated simultaneously fo sensing of multiple paametes. The ecent attempts have been towads fabicating photonic senso chips fo fast and eliable detection of biological species [83]. Howeve, these sensos chips can be of much benefit if collaboated with optical fibe [84]. They can then be used fo online and fast detection, emote sensing and multi-paamete sensing. Plasmonic senso chips with optical fibe integation seem to be futue senso devices. Apat fom that, some ecent epots show a huge enhancement of pefomance of vaious nanophotonic sensos and whispeing galley mode (WGM) esonatos [85].The whispeing galley modes ae geneally excited by the evanescent field of tapeed optical fibes. The WGM sensos have been found to have vey low detection limit, up to single molecules. When collaboated with plasmonic stuctues, they ae seen to detect single molecule inteactions moe accuately and pecisely [86]. APPENDIX A Suface Plasmons: Mathematical Famewok Suface plasmons ae the modes suppoted by a metaldielectic inteface. These ae basically suface waves, which get geneated by the cumulative action of conduction electon oscillations and incident electomagnetic field. These waves tavel along the metal-dielectic inteface and thei field amplitude decays exponentially in both the media. As shown in Fig. (1), if thee be a metal-dielectic inteface along the z-diection, then the SP wave popagates along the x axis. A complete teatise about suface plasmons can be found in ef. [87]. Howeve, we pesent below a bief and simple mathematical famewok of suface plasmons. Let the efactive index of a dielectic medium be! s and that of the metal be! m =! 1 + i! 2, whee! 1 and! 2 ae espectively the eal and imaginay pats of the metal dielectic function. The electic and magnetic field components of the fou Maxwell s equations ae witten and then the bounday conditions ae satisfied to obtain the modal fields. Fom the modal fields suppoted by the inteface, the expessions fo popagation constant, popagation length and penetation depth ae deduced. The Maxwell s equations ae given as:!. D = " ext (A.1)!. B = 0 (A.2)! " E = # $ B $t (A.3)! " H = J ext + # D (A.4) #t whee D, B, E and H epesent the fou macoscopic fields: dielectic displacement, magnetic induction, electic field and magnetic field espectively;! ext and J ext denote the extenal chage and cuent densities espectively. Fo time vaying hamonic fields (! = "i# ), the!t Maxwell s equations can be eaanged to give two sets of self-consistent solutions having diffeent polaizations. One set of solutions is called the tansvese electic (TE o s) mode and othe set tansvese magnetic (TM o p). Fo the kind of geomety shown in Fig. (1), the set of (H x, E y and H z ) coesponds to TE mode and the set of (E x, H y and E z ) coesponds to TM mode. The coesponding TE and TM wave equations ae given by equations (A.5) and (A.6).