A Lossy Mode Resonance optical sensor using silver nanoparticles loaded ﬁlms

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A Lossy Mode Resonance optical sensor using silver nanoparticles-loaded filmsfor monitoring human breathing

PedroJ. Rivero, A. Urrutia,J. Goicoechea, I.R. Matias, F.J. Arregui

Electric and Electronic Engineering Department, Public University of Navarra, Campus Arrosada, 31006Pamplona, Spain

a r t i c l e i n f o

Article history:

Available online xxx

Keywords:

Silver nanoparticles (Ag-NPs)

Lossy ModeResonances (LMR)

Humidity sensing

a b s t r a c t

Thiswork is focused onthe fabrication ofa humanbreathing sensor based onthe in situ synthesis ofsilver

nanoparticles (Ag-NPs) inside a polymeric coating previously deposited on an optical fiber core bymeans

of the Layer-by-Layer self-assembly. The Ag-NPs were created using a synthesis protocol consisting ofa

loading step ofthe Ag+ cations into the polymeric film anda further reduction step using dimethylamine

borane (DMAB). The morphology and distribution ofthe Ag-NPs inside the polymeric coating have been

studied using atomic force microscopy (AFM). Furthermore, UVVIS spectroscopy and energy dispersive

X-ray (EDX) were also used to confirm the synthesis of the Ag-NPs within the resultant coating. The

amount ofAg-NPs increases when the number ofloading/reduction cycles is higher. Therefore the incor-

poration ofthe Ag-NPs affects the refractive index ofthe overlay promoting the observation ofa resonant

attenuation band in the infrared region (NIR), known as LossyMode Resonance (LMR), which can be used

as a sensing signal to monitor the human breathing. The quality ofthe device has been experimentally

tested with good sensitivity (0.455nm per RH%) and fast response time (692msand 839ms for rise/fall).

2012 Elsevier B.V. All rights reserved.

1. Introduction

During the last two decades a growing attention has been

paid to the Layer-by-Layer assembly (LbL) due to the potential

applications in electronics and sensing devices. Some of the key

points of thismethod are its simplicity, versatility and easiness for

scaling-upwith a precise control of the thickness [1,2]. Due to this,

the LbL technique has been applied to a wide range of polymers

and nanomaterials (polyelectrolytes, nanoparticles or nanocom-

posites) [35] for diverse applications (anti-reflection, anti-fog,

anti-corrosion, hydrophilic or hydrophobic coatings) [69]. These

coatings can be used to fabricate optical fiber sensors (small size,

simple geometryor biocompatibility)which havebeenconsidered

as a good choice for obtaining sensors based on LbL sensitive films

[10]. Moreover, the useof optical fiber sensors hasbeen increasing

forhumidity controldueto provide several advantages such assen-

sitivity, fast response, dynamic range or remote sensing capability

[11,12].

Very recently, a novel optical fiber humidity sensor based on

both lossy-mode resonance (LMR) and Localized Surface Plasmon

Resonance(LSPR)hasbeenexplored [13]usingLbL polymericcoat-

ing loaded with silver nanoparticles (Ag-NPs). The utilization of

Corresponding author. Tel.: +34 948 16 60 44; fax: +34 94816 97 20.

E-mail address: [email protected](P.J. Rivero).

metal nanoparticles (Ag-NPs) for optical sensors is well-known

due to the phenomenon of LSPR as a sensing signal [1419] and

the location of this resonant peak in the visible region depends on

multiple factors such as nanoparticle size, shape or particles inter-

action. However, a newtypeof resonances (LMR)canbe supported

by thin-film coated optical waveguides. These optical resonances

occur when the real part of the thin film permittivity is positive

and higher inmagnitude than both its own imaginary part and the

real part of the material surrounding the thin film [20,21]. LMR-

baseddevices havebeenexploredforthe fabricationofoptical fiber

refractometers [2224], humidity sensors [25,26] or pH sensors

[27]. Thedevicesbasedon LMRattenuationbands canbegenerated

using diverse materials such as polymers or ceramics. Further-

more,LMR-baseddevicesmakepossible thegenerationof multiple

absorptionbandswithoutmodifying theopticalfibergeometryand

it is possible toobtainsensing signals as a functionof the thickness

and refractive index of optical fiber overlay.

In addition to this, very recently a new route of synthesis ofin

situ Ag-NPs has been developed for antibacterial applications [28].

It is important to remark that the use of Ag-NPs has an additional

advantagewhen the sensors are placed in high humidity environ-

ments due to their antibacterial behavior preventing the bacteria

growth [29,30]. In this work, a polymeric supporting layer was

modified by in situ Ag-NPs synthesis. This synthetic route is based

ona first Ag+ loading step followedbya further reductionstep. Ag-

NPsmake possible an increasing of therefractive indexandpermit

0925-4005/$ seefrontmatter 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.snb.2012.09.022
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the presence of a new optical resonant band (LMR) in the infrared

region (9001600nm) which it is used as a sensing signal. To our

knowledge, this is the first time that this new route of synthesis of

in situ Ag-NPs has been described in literature to achieve optical

fiber Relative Humidity (RH) sensors.

2. Experimental process

2.1. Materials

Poly(allylaminehydrochloride) (PAH) (Mw56,000),poly(acrylic

acid, sodium salt) 35wt% solution in water (PAA) (Mw 15,000),

silver nitrate (>99% titration) solution in water 0.1N and

borane dimethylamine complex (DMAB) were purchased from

SigmaAldrich andusedwithout any further purification. Aqueous

solutionsof 10mM of PAHandPAAwere prepared usingultrapure

deionizedwater (18.2M) and adjusted to pH 7.0 by the addition

ofa fewdrops ofNaOHorHClwithno additional salt concentration.

Plastic-clad silica fibers of 200/225m core/cladding diameter

(FT200EMT) were provided by Thorlabs Inc.

2.2. Device fabrication

2.2.1. Fabrication of the polymeric matrix

A 4cm long part of the optical fiber cladding was chemically

removed in order to expose part of the optical fiber core prior to

the coating fabrication.

A polymeric matrix has been synthesized using the Layer-by-

Layer technique (LbL) by sequentially exposing the optical fiber

coreto thecationicpolyelectrolyte poly(allylamine hydrochloride)

(PAH) and to the anionic polyelectrolyte poly(acrylic acid, sodium

salt) (PAA) with an immersion time of 2min in each solution. A

rinsing step in deionized water was performed between the two

polyelectrolytes baths in order to remove the excess of material

adsorbed.After this sequence,it hasbeen fabricated the[PAH/PAA]

basic structure, known as bilayer. This process was carried out

using a 3-axis robot (Nadetech Innovations)andwasrepeateduntil

reaching a [PAH/PAA] structure of 40 bilayers.

2.2.2. In situ synthesis of silver nanoparticles into the polymeric

coating

Once the polymeric overlay was deposited on the optical fiber

a novel method for the in situ synthesis of Ag-NP was applied in

order to incorporate these Ag-NPs in the polymeric matrix [28].

Basically, the Ag-NPs havebeen synthesized in theLbL coating bya

two step synthesis process. Firstly, anAg+ cation loadingstepusing

silver nitrate as a loading agent which formed electrostatic pairs

with some of the carboxylate groups from PAA. Secondly, a further

reduction of the silver loaded into the coating by immersing into

a dimethylamine borane(DMAB) solutionwhichacted as reducing

agent. The carboxylate-bonded Ag+ cations have been reduced to

produce zero-valent silver nanoparticles (Ag0).This loading/reduction cycle has been repeated up to 6 times.

These Ag-NPs loaded LbL overlays onto the optical fiber core will

be the sensitive region, as it is schematically shown in Fig. 1.

2.3. Device characterization

UV/Vis absorbance spectroscopy was used to monitor both the

polymeric coating process and the in situ Ag-NPs fabrication pro-

cess. The experimental set-up consisted of a white halogen lamp

(ANDO Inc.) used as the excitation source which was connected

to one end of the optical fiber and a CCD-based NIR spectrome-

ter (NIR512 from Oceanoptics Inc.) which was connected to the

other end of the fiber in order toobtain spectral information in the

rangebetween900and1600nm.Light passes throughthe sensitive

Fig.1. Schematicrepresentationof thedevice basedon Ag-NPs loaded LbLoverlays.

regionwhich is located between the light source and the detector,

and it is modified with the new boundary conditions created by

the polymeric coating and the incorporation of the Ag-NPs. After

fabrication of the sensor, the same setup was used to characterize

thedevicewhen it is subjected to RelativeHumidity (RH) changes.

An environmental chamber (Angelantoni Inc.) was used to control

both RH and temperature surrounding the sensor.

2.4. Characterization of the synthesized Ag-NPs

EnergydispersiveX-ray (EDX) anddiffractionX-ray (XRD)were

used to confirmthe presence of the crystalline silvernanoparticleswithin the polymeric coating. The measurements were obtained

from an INCA-X-ray microanalysis system from Oxford Instru-

ments.

Atomic force microscopy (AFM) was also used to character-

ize the distribution and the surface morphology of the resultant

coating. The samples were scanned using a Veeco Innova AFM, in

tappingmode.

3. Results anddiscussion

Asithas been reported inpreviousworks[2027], differenttype

of selective optical power absorption at certain wavelengths, can

be supported by thin-film coated optical fiber core and the nature

of these resonances depends on theoptical properties of the outerthin-film surrounding the optical waveguide. SPRoccurswhen the

real part of thethin-filmpermittivityis negativeandhigherinmag-

nitudethanboth itsown imaginarypart and thepermittivity of the

material surrounding the thin-film. LMRoccurswhen the real part

of the thin-film permittivity is positive and higher in magnitude

than both its own imaginary part and the material surrounding

the thin-film. In fact, devices based on ITO, TiO2 or InO2 coatings

havebeen alreadyexperimentallydemonstrated[2224]. Here, the

main goal of thiswork is toprove experimentally thegeneration of

LMRby means of the in situ synthesis of Ag NPs into a polymeric

overlay and its utilization for humidity sensing purposes. In the

next paragraphs, different techniques of analysis have been used

to corroborate the presence of AgNPs in the film and also that this

coating can produce LMRabsorption bands in the infrared regionwith a high wavelength dependence to relative humidity changes.

More details about the theoretical basis of LMR-based sensors can

be found in the literature [3133].

It is well-known that the PAH/PAA structure fabricated onto

the core of the optical fiber presents a thickness dependencewith

theRHof thesurroundingmedium, knownas swelling/deswelling,

which it has been exploited before in the fabrication of humid-

ity or pH sensors [26,27]. The use of Ag-NPs which were further

in situ synthesized formsilver nitrate (loadingagent),and dimethyl

borane complex (reducing agent) inside the previously polymeric

PAH/PAA coating obtained by LbL assembly plays a key role in the

modificationof therefractive index. TheseAg-NPswhichweresyn-

thesized increaseboththe realandimaginary components,making

possible to observe stronger LMRbands in the infrared region.
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Fig. 2. AFM figure (20m20m) in tapping mode of height (left) and phase

(right) respectively.

A random distribution of the synthesized Ag-NPs inside the

polymeric coating is confirmed by AFM analysis (see Fig. 2). The

light spots observed in the topographic image (Fig. 2, top) corre-

spond to the Ag-NPs. This is supported by the phase image (Fig. 2,

bottom) that also confirms the presenceofmaterialswith different

stiffness, such as thepolymericmatrix and the metallic nanoparti-cles.

In Fig. 3a, it is possible to appreciate an absorption band near

450nm due to the LSPR phenomenon, typical of metallic silver

nanoparticles. The visual aspect of the thin film synthesized onto

the optical fiber core after the loading/reduction cycles showed a

dramatic color change to golden-yellowish. This colored appear-

ance is the result of the presence of a LSPR absorption band in the

410450nm region, dueto thepresence of the silvernanoparticles

within thepolymericoverlay. Thepositionand intensity of theLSPR

absorption band depends onmultiple factors such as shape, size or

distribution of them in the polymeric film.

The size and amount of the Ag-NPs is increased when the num-

ber of load/reduction dips is increased. In Fig. 3b, it is shown the

relation between the number of loading/reduction cycles and theincreasing of the intensity of the LSPR absorption band, propor-

tional to the amount of Ag-NPs trapped into the thin film. An

exponential-like growth of the intensity of the LSPR absorption

band was observed with the increasing of the consecutive load-

ing/reduction cycles. Additional techniques not shown here (EDX

or XRD)were also used to confirm the presence of suchAgNPs in

the final coating.

These Ag-NPs loaded LbL overlays on optical fiber present sev-

eral absorption bands in the spectral range from 400 to 1600nm

when their transmission spectra were monitored. The first one

appears at 450nm (visible region) after the loading/reduction

cycles which it is due to the SPR of the synthesized Ag-NPs.

Furthermore, a new optical resonant band (LMR) is observed

in the infrared region (NIR). The amount and size of the metal

Fig. 3. (a) UVVIS absorption spectra of the coating with different number of

loading/reduction cycles(top) and (b) maximum absorbance of the SPR of the syn-

thesized Ag-NPs as a function of thenumber of loading/reduction cycles (bottom).

nanoparticles in theLbL coatingmodifies the refractive indexof thefilmand consequently the wavelength of the LMRbandmaximum

is shifted to longer wavelengths as more loading/reduction cycles

were performed (Figs. 4 and 5). In previous work, it has been

experimentally demonstrated the high wavelength shift of the

LMR attenuation bands, compared to the low sensitivity of the

SPR absorption band [13]. Due to this, it has been studied the

Fig.4. Spectral responseat NIRas a functionof thenumberof theloading/reduction

cycleson theoptical fiber core.


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Fig.5. Evolutionof theUVVISabsorptionspectrawith 4, 5 and6 loading/reduction

dips (L/R) at theLMR wavelength.

wavelength dependence of the LMRband in the infrared region

with relative humidity changes (Fig. 6).

It is important to remark that the in situ loading/reduction syn-

thesis routemakes possible themonitoring of theoptical response

of the optical fiber during the whole synthesis process. This allowsus to stop the Ag-NPs growthwhen the LMRband is located at the

desired wavelength in the infrared region, 1150nm in this case.

Once the sensorwasfabricated in order to position theworking

point of the sensor in the sensitivity region (NIR), the device was

tested to cyclic variations from 20 to 80% Relative Humidity (RH)

changes with a time of 3h for rise/fall step respectively. In Fig. 6

the dynamical response of LMRto different RH values at constant

temperature (25C) forseveral cycles is shown. Thedynamic range

of the sensor in the studied range is 27.3nm which corresponds

to a sensitivity of 0.455nm per RH%. In addition, the resonance

wavelength shift follows perfectly the RHmeasurements from the

electronic sensor located in theclimaticchamber. Furthermore, the

sensorhas been testedseveral times to thesameRH cycleswithout

any significant changes in the optical response. These tests havebeen performed in different periods of time with the same cyclic

variations from 20 to 80% Relative Humidity (RH) and the results

show a slight change of less than 3% in intensity and less than 1%

in wavelength shift.

Finally, the performance of the device has been tested exper-

imentally for human breathing changes at the LMRwavelength

to evaluate the response time. The results of this experiment to

thesequick changes of RHmeasurements areshown in Fig. 7a. The

observed response time of the sensor was 692 and 839ms for the

Fig. 6. Dynamic response of the device (LMR maximum sensitivity) to RH changes

from20 to80%at 25

C.

Fig. 7. Response of thesensor to several consecutive human breathingcycles (top)and (b) response time of thesensor for therise and fall, respectively (bottom).

rise and fall, respectively (Fig. 7b). As the result, the combination

of nanotechnology and biomedical science makes enable their use

in practical RH monitoring applications or even, can lead to mon-

itor high humidity changes such as human breathing (biomedicaldevices),due tothefast responsein themaximumsensitivityregion

at 1150nm (NIR).

4. Conclusions

A lossy-mode resonance optical sensor based on wavelength-

displacement in the infrared region (NIR) has been fabricated.

Firstly, a polymeric matrix has been obtained using the Layer-

by-Layer technique (LbL) and then Ag-NPs have been synthesized

in situ in the LbL coating by consecutive loading/reduction cycles.

The synthesis of theseAg-NPs hasbeen corroborated using several

techniques such as UVVIS, EDXor XRD. Furthermore, AFManaly-

siswasused to show a randomdistributionof synthesizedAg-NPs

in the resultant polymeric coating.These Ag-NPs allow us to fabricate LbL films with modified

refractive index. In addition, Ag-NPs play a key role due to the

generation of a new LMRband at 1150nm in the NIRwith a high

wavelength shift as a function of thenumber of loading/reduction

cycles. In addition, the device has been tested to RH changes with

a sensitivity of 0.455nmper RH% and a dynamic range of 27.3nm.

Finally, this LMRband could be used as a sensing signal with a

very fast response time (692ms and 839ms for rise/fall respec-

tively) and a good repeatability to several exhalation/inhalation

cycles. To our knowledge, this is he first time that this new route

of synthesis ofin situ Ag-NPs has been reported to monitor quick

changes ofhumiditysuchas humanbreathing in the literature. The

development of this LMR-sensing configuration could be designed

forthemeasurementof physical,chemicalorbiologicalparameters.


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A Lossy Mode Resonance optical sensor using silver nanoparticles loaded ﬁlms

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