Love wave sensors based on gold nanoparticle-modified polypyrrole and their properties to ammonia and ethylene

Con en s lis s a ailable a ScienceDi ec Senso s and Ac ua o s B: Chemical jou nal homepage: www.else ie .com/loca e/snb Lo e wa e senso s based on gold nanopa icle-modified polypy ole and hei p ope ies o ammonia and e hylene M. Še ka a , F.A. Bahos b , D. Ma a agui b , M. Po oček a , Z. K al d , J. D bohla o á a , I. G àcia c , S. Vallejos a,c, * a CEITEC - Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, 61200, B no, Czech Republic b Ins i u o de Ciencias Aplicadas y Tecnología (ICAT), Uni e sidad Nacional Au ónoma de México, Ciudad Uni e si a ia, Ciudad de México, 04510, Mexico c Ins i u o de Mic oelec ónica de Ba celona (IMB-CNM, CSIC), Campus UAB, 08193, Bella e a, Spain d The mo Fishe Scien ific, Analy ical Ins umen s - Ma e ials and S uc u al Analysis, Hillsbo o, OR 97123, USA ARTICLE INFO Keywo ds: Gas senso s SAW senso s Polypy ole ABSTRACT This wo k p esen s he ab ica ion and gas sensing p ope ies o Lo e wa e senso s based on non-modified and gold nanopa icle-modified polypy ole. Resul s demons a e he in eg a ion o uni o m polypy ole laye s wi h smoo h g anula su ace and he inco po a ion o dispe sed c ys alline gold nanopa icles wi hin he modified laye s. Gas sensing es s o he senso s in d y condi ions show enhanced sensing pe o mance o e hylene and ammonia o he modified sys ems as compa ed o hose wi hou modifica ion and hose epo ed in he li - e a u e. The effec o humidi y p o es significan in bo h sys ems wi h he esul s showing a dec ease o sen- si i i y in humid condi ions. Despi e his weakness, he ela i ely acile and scalable ab ica ion o hese senso s, as well as hei sensing esponse a oom empe a u e may be a ac i e in gas de ec ion sys ems, in which high humidi y le els can be es ic ed by he use o fil e o p econdi ioning elemen s. 1. In oduc ion O e he las decades, piezoelec ic acous ic de ices ha e gained eno mous in e es o senso applica ions, including gas senso s, be- cause o hei ope a ion a oom empe a u e (RT), high sensi i i y, low limi s o de ec ion (LOD), ela i ely easy ab ica ion and low cos . Agains a hos o compe ing echnologies o piezoelec ic acous ic senso s, su ace acous ic wa e (SAW) based gas senso s ha e gained mo e in e es han hei bulk acous ic wa e (BAW) coun e pa s, gen- e ally due o hei highe ope a ing equency and in u n highe sen- si i i y [1]. In SAW senso s, he p opaga ion o acous ic wa e (ei he Rayleigh, Shea Ho izon al SAW, Lo e, Leaky, S oneley, o Lamb wa es) is lim- i ed o he su ace and uned in pa by he cu o he piezoelec ic c ys al [[2]]. In pa icula , Lo e SAW (L-SAW) based senso s allow effec i e guidance o he wa e close o he sensing su ace by confining he wa e ene gy in a hin guiding laye . This slows he eloci y o he wa e wi h espec o he piezoelec ic subs a e and make he su ace highly sensi i e o small pe u ba ion [3]. To his end, elas ic laye s such as SiO 2 a e commonly used on he op o piezoelec ic subs a es as guiding laye s (pa icula ly due o hei low acous ic loss), bu hey canno achie e a e y high sensi i i y due o shea s iffness. Howe e , o he ma e ials, such as polyme s, wi h slowe ans e se wa e eloci y and lowe densi y han SiO 2 (due o hei iscoelas ic p ope ies) can also se e as guiding laye s, p o iding u he ad an ages in e ms o sensi i i y, al hough wi h g ea e p opaga ion loss han SiO 2 . In his con ex , a mul iguiding laye concep , in which elas ic and iscoelas ic p ope ies o SiO 2 and polyme a e balanced is gene ally mo e ad- an ageous [4]. The Lo e wa e p opaga ion and he a enua ion a e s ongly dependen on he p ope ies o he guiding laye , including he hickness and s iffness, hus wi h he op imal hickness o he mul i- guiding laye s is possible o achie ed he high sensi i e L-SAW senso s wi h app op ia e a enua ion o he wa e [3,5]. Addi ionally, he spe- cific use o gas sensi i e polyme s in he mul iguiding laye , which in e ac physically/chemically wi h he analy es, can also p o ide u - he enhancemen in e ms o sensi i i y and selec i i y ia polyme modifica ion o unc ionaliza ion [1b,2b]. Polypy ole (PPy) is a conduc i e polyme ha has p o ed sensi- i i y o a ious gases and o ganic apou s, including ammonia, e hanol, and ace one [6]. In gene al, PPy was in eg a ed in chemi - esis i e senso s, and so a , only ew examples o he use o PPy in SAW senso s we e epo ed in he li e a u e. These examples include SAW senso s based on Rayleigh wa es o de ec ammonia [[7]], ace one[7c], NO 2 [8] and H 2 S[8], o shea ho izon al SAW (e.g., Leaky) o de ec h ps://doi.o g/10.1016/j.snb.2019.127337 Recei ed 9 July 2019; Recei ed in e ised o m 19 Oc obe 2019; Accep ed 23 Oc obe 2019 ⁎ Co esponding au ho a : CEITEC - Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, 61200, B no, Czech Republic. E-mail add esses: [email p o ec ed],[email p o ec ed] (S. Vallejos). Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 A ailable online 30 Oc obe 2019 0925-4005/ © 2019 The Au ho (s). Published by Else ie B.V. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/BY-NC-ND/4.0/). T hyd ogen [9]. Acco ding o published esea ch, we ound ha he u i- liza ion o PPy in L-SAW senso s is less common. Howe e , ou ecen p elimina y esul s [10] showed he po en ial o PPy-based L-SAW senso s o de ec low concen a ions o ammonia. Ce ainly, he syn hesis ou e o PPy defines i gas sensing p ope - ies, and al hough he e a e a ious a ailable elec ochemical o che- mical ou es o he syn hesis o nanoscaled PPy, he in eg a ion o PPy wi h SAW pla o ms using chemical ou es is ye mo e scalable and ela i ely easie . Fu he mo e, chemical ou es also acili a e he unc ionaliza ion o PPy wi h di e se nanosized ma e ials, including noble me als, ca bon-based ma e ials o me al oxides. The po en ial applica ion o L-SAW senso s in ea ly disease diag- nosis may be p omising due o hei high sensi i i y and low LOD [1b], which a e in he ppb ange as hose o specific gas bioma ke s con- ained in human b ea h [11]. Fo ins ance, e hylene and ammonia ha e ound o be ele an bioma ke s o oxida i e s ess (OS), a condi ion ha damages he cells in human body and can lead e en ually o ch onic diseases such as a he oscle osis, cance , diabe es, ca dio as- cula , neu odegene a i e and o he degene a i e diseases in humans [12]. This condi ion is gene ally caused by he p esence o eac i e oxygen species (ROS) in he body [13], which cause oxida i e de- g ada ion (lipid pe oxida ion) o polyunsa u a ed a y acids and hus elease o e hylene in human b ea h [14]. The inc emen o ROS, mo eo e , has ecen ly ound o be also connec ed o he accumula ion o u emic oxins caused by kidney dys unc ions and in u n o he e- lease o ammonia gas in exhaled b ea h [15]. In his con ex , he p esen s udy epo s he ab ica ion o mul iguiding L-SAW senso s based on SiO 2 and gas sensi i e PPy na- nopa icles (NPs) unc ionalized wi h gold NPs. The wo k deepens in o he p ope ies o he second guiding laye and he influence o wo diffe en gold loadings on he sensing unc ionali y owa ds low con- cen a ions o ammonia and e hylene as po en ial gaseous bioma ke s. 2. Expe imen al 2.1. Syn hesis o PPy and Au/PPy NPs PPy NPs we e ob ained ia oxida i e chemical polyme iza ion o py ole monome , as desc ibed p e iously [16]. In he fi s s ep, poly- inyl alcohol (PVA, 7.5 g) was dissol ed in 92.5 ml o deionize wa e , Fig. 1. (a) (b) Schema ic iew o he L-SAW sensing elemen . SEM-image o he PPy guiding/sensi i e laye modified wi h Au NPs a e spin coa ing; no ice he diffe ence in b igh ness/con as o he PPy g ains and he Au NPs. Fig. 2. UV– is analysis o (a) PPy NPs, (b) Au NPs, (c) Au/PPy NPs (1:10, black colo ) and Au/PPy NPs (10:1, g een colo ). (Fo in e p e a ion o he e e ences o colou in his figu e legend, he eade is e e ed o he web e sion o his a icle). M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 2 whe eupon 3.73 g o i on ca ions (III) o FeCl 3 we e added in he so- lu ion and s i ed o 10 min. In second s ep, py ole (0.01 mol) was added o his aqueous solu ion and s i ed o 5 h a RT. As soon as py ole was mixed wi h he oxidan (FeCl 3 ), a apid polyme iza ion eac ion occu ed, which u ned he solu ion in o a cha ac e is ic black colo indica ing he o ma ion o PPy. Colloid Au NPs we e syn hesized acco ding o Tu ke ich me hod [17], i.e., in a we chemical syn hesis, ia he educ ion o gold sal by sodium ci a e. B iefly, 20 mL o 0.001 M o gold (III) chlo ide ihy- d a e (HAuCl 4 ·3H 2 O) was added o a flask and placed on a s i ing ho pla e a 150 °C. As soon as he solu ion each i s boiling poin , 2 mL o 1% sodium ci a e dihyd a e (Na 3 C 6 H 5 O 7 *2H 2 O) was added and empe a u e was dec eased o 100 °C. The eac ion was un un il he solu ion became eddish. A e wa ds he solu ion was emo ed om he ho pla e and cool down. Au/PPy NPs we e p epa ed by mixing Au and PPy NPs solu ions wi h wo diffe en a ios, 1:10 and 1:2. The Au/PPy NPs solu ion was sonica ed in ul asonic ba h o 10 min and used immedia ely, ei he o i s cha ac e iza ion o o spin coa ing on he L-SAW subs a es o sensing es . 2.2. Ma e ial cha ac e iza ion The abso p ion spec a o PPy, Au and Au/PPy NPs we e measu ed a oom empe a u e using UV– is-NIR spec opho ome e (Ca y 5000) in he ange o 200–800 nm. The NPs solu ions we e measu ed in 1 cm op ical pa h qua z cu e e. The mo phology and size dis ibu ion o he PPy, Au and Au/PPy NPs we e examined using High Resolu ion T ansmission Elec on Mic oscope (HR-TEM, FEI TITAN Themis 60–300 kV wi h Cs image co ec o ) a accele a ing ol ages o 60 kV o PPy and Au/PPy NPs and 300kV o Au NPs. The samples o HR- TEM analysis we e p epa ed by placing a d op o he co esponding NPs solu ion on o TEM holey ca bon holey g ids. 2.3. Lo e-wa e senso s ab ica ion L-SAW delay line pla o ms consis ing o a piezoelec ic subs a e (ST-90°X qua z, 9 mm × 4 mm × 0.5 mm) wi h wo (inpu /ou pu ) aluminium in e digi a ed ansducing (IDTs, 200 nm hick) po s, see Fig. 1. The double finge pai IDTs ha e ou s ips pe pe iod (λ=28μm) and his s uc u e is epea ed 75 imes o each po . The IDTs ape u e and dis ance be ween IDTs po s (delay line) a e 2.1 mm. The fi s guiding SiO 2 laye (3 μm hick) was deposi ed on he op o subs a e. Addi ionally, SiO 2 laye se ed as an isola ing laye be ween IDTs and second guiding laye . The whole ab ica ion p ocess consis ed o a ious mic o ab ica ion s eps, including me alliza ion, oxide de- posi ion and li hog aphy; u he de ails o he ab ica ion p ocess we e epo ed p e iously [18]. The second guiding/sensi i e laye (350 ± 50 nm hick) was spin- coa ed o e he fi s (SiO 2 ) guiding laye a a speed o 4000 pm and accele a ion o 4000 pm/s o 1 min. To con ol he ep oducibili y o he spin coa ing, he L-SAW subs a es we e placed in o a cus omized holde buil o keep he subs a es in a fix posi ion and alignmen du ing he coa ing. Mo eo e , shadow masks we e used o co e he elec ode con ac s placed in he ou side co ne s. Subsequen ly, he delay line and IDTs po s a ea was co e ed wi h 60 μl o he co e- sponding solu ion (i.e., PPy, Au/PPy (1:10) o (1:2)) and spin coa ed. The edge effec (s udy by p ofilome y) nea o he elec ode con ac and hei possible wa e eflec ion was con olled by applying a hin film o silicon (d ied a 60 ºC o 1 h) a he ou side co ne s. To ab ica e he L-SAW de ice wi h app op ia e wa e a enua ion and p e en high noise in he oscilla o sys em, he hickness o fi s and second guiding/sensi i e laye we e uned expe imen ally. Resul s o a ious sys ems, including SiO 2 laye s wi h 1.6, 2.5, 3, and 3.8μm and PPy o AuPPy spin coa ed a a ious pm ( om 2000 o 4000), showed a good comp omise in e ms o losses (∼20 dB) o he L-SAW senso s comp ising a 3 μm SiO 2 laye . The e o e, hese sys ems we e employed o u he ma e ial and gas sensing es s. 2.4. Lo e-wa e senso s cha ac e iza ion The mo phology o he second guiding laye (PPy, Au/PPy (1:10) and (1:2)) a e hei in eg a ion wi h he L-SAW subs a es we e cha ac e ized using FIB/SEM Mic oscope (Helios G4 NanoLab DualBeam™) and Scanning P obe Mic oscope (B uke Dimension Icon, wo king in ScanAsys -Ai Type mode). The L-SAW senso s we e elec- ically cha ac e ized be o e and a e he in eg a ion o PPy o Au/PPy NPs using RF ansmission pa ame e S 21 (360B Au oma ic Ne wo k Fig. 3. HR-TEM images o he (a) PPy NPs and (b) Au/PPy NPs wi h (c) close iew on he Au NPs. Size dis ibu ion o he (d) PPy NPs and (e) Au NPs es ima ed o a popula ion o 100 and 45 pa icles, espec i ely. M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 3 Analyze , Wil on) o analyse he inse ion loss o each sample. All es ed L-SAW senso s we e analysed by Time-o -Fligh Seconda y Ion Mass Spec ome y (ToF-SIMS) using TOF.SIMS5 (Ion-To ). ToF-SIMS combined wi h spu e dep h p ofiling is used o p o ide he in o ma- ion o he su ace chemical composi ion, whe e laye -by-laye spu - e ing allows econs uc ing elemen al and molecula dis ibu ion by sample dep h [19]. A c a e wi h a ea o 300 × 300 μm 2 was spu e ed by Cs + beam (1 keV). Elec on flood gun was used o educe cha ging o he sample. The pulsed Bi + wi h accele a ion ol age o 30 keV was used as he p ima y ion in o de o collec he seconda y ion mass spec a.The seconda y ions emi ed om he su ace o he c a e bo om we e de ec ed in nega i e ion mode by he TOF mass spec o- me e . The analyses we e ob ained om a eas o 100 × 100 μm 2 . The ins umen was uned o a mass esolu ion g ea e han 4000 a m/z = 29 Si. 2.5. E hylene and ammonia es s The gas sensing p ope ies o he L-SAW senso s we e es ed a RT (24 °C) in a con inuous gas flow es chambe . The es s consis ed in moni o ing he equency changes o he senso s owa d a ious e hy- lene (P axai ) and ammonia (P axai ) concen a ions (2, 5 and 10 ppm) Fig. 4. (a) Top iew SEM image and (b) 3D AFM image o he second guiding/ sensi i e laye based on Au/PPy NPs a e spin coa ing. (c) Roughness p ofile o he laye analysed om he AFM images. Fig. 5. (a) Nega i e SIMS spec um o Au - ion in he dep h p ofile o PPy and Au/PPy (1:10 and 1:2) guiding/sensi i e laye s. (b) Co ela ion o Au NPs loadings and in ensi y o seconda y Au - ions in PPy and Au/PPy (1:10 and 1:2) guiding/sensi i e laye s. Fig. 6. Elec ical esponse egis e ed o he e e ence sample (uncoa ed, wi h only SiO 2 guiding laye ) and he L-SAW senso s con aining he second guiding/ sensi i e PPy laye . M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 4 using syn he ic d y ai as dilu ing and ca ie gas. Fu he es s o he senso s in humid ambien we e pe o med a 10 and 30 % RH as hese a e he humidi y le els eached in ‘ac ual’b ea h samples a e p e- condi ioning and fil e ing mois u e [20]. Bo h, empe a u e and hu- midi y we e moni o inside he es chambe using a humidi y/ em- pe a u e senso (SHT71, ope a ing anges om 0 o 100 % ela i e humidi y (RH), accu acy o ± 3 % RH). Each L-SAW senso wo ks in an oscilla o ci cui , which includes an amplifie and a di ec ional couple . The e o e, he oscilla ing equency o he senso is shi ed o any pe u ba ion. In addi ion, he amplifi- ca ion s age o he oscilla o is con olled by a heos a de ice, which op imizes he gain o he oscilla o o each senso , o p e en ha he inse ion losses exceed he amplifie gain du ing he gas expe imen . The measu emen s o he equency o a pe iod o wo minu es gen- e ally displayed good s abili y wi h he noise do no exceeding 10 Hz. A he e odyne configu a ion was used o signal acquisi ion, mixing he signal o he oscilla o coupled o a e e ence L-SAW senso (wi h only SiO 2 guiding laye ) and he signal o he oscilla o coupled o he PPy based L-SAW senso . The equencies ob ained om he mixe we e acqui ed by a equency coun e . The senso s we e placed in a con- inuous flow es chambe (100 mL/min) equipped wi h mass-flow con olle s. The exposu e ime o he senso s o each analy e con- cen a ion was se o 2 min, wi h a subsequen pu ging using syn he ic d y ai o 30 min. The e e ence L-SAW senso was used in he gas sensing a ay, o compensa e he mal and o he en i onmen al depen- den d i s on he piezoelec ic subs a es. Du ing he gas es , he e- e ence elemen was no exposed o a ge gas analy es, and he final ou pu signal o he L-SAW senso s was sub ac ed om he e e ence sample. The e o e, he senso esponse was defined as he equency shi , which is he di e gence o senso s equency ead offdu ing he exposu e o syn he ic ai and gas. 3. Resul s and discussion 3.1. Cha ac e iza ion o he PPy and Au/PPy NPs Fig. 2 displays he UV– is spec a o he syn he ized PPy, Au, and Au/PPy NPs; he spec um o Au/PPy (10:1) mix u e, i.e., wi h 10 imes highe olume ic amoun o Au NPs han PPy NPs, is also included in Fig. 2c o compa a i e pu pose. The UV– is spec um o PPy NPs (Fig. 2a) shows a b oad abso p ion peak a 461 nm assigned o π–π* elec on ansi ion om alence band o he conduc ion band due o he p esence o pola ons [21]. The UV– is spec um o he Au NPs (Fig. 2b) is consis en wi h he li e a u e showing he maximum o su ace plasmon esonance abso p ion a 523 nm and indica ing he o ma ion o NPs wi h diame e s be ween 15 and 20 nm [22]. The analysis o he Au/PPy NPs wi h diffe en Au loadings (i.e., 1:10 and 1:2) showed si- mila spec a o ha eco ded on he non-modified PPy NPs (Fig. 2a). Howe e , he compa ison o he spec a o Au/PPy NPs (1:10, Fig. 2c- black line) and Au/PPy NPs (10:1, Fig. 2c - g een do line) demons a es ha he shape and posi ion o he abso p ion maximum depends on he dominan olume ic quan i y. The e o e, he appa en absence o he Au plasmon peak a 523 nm in he Au/PPy NPs (1:10 and 1:2) solu ions is connec ed wi h he low amoun o Au NPs in hese mix u es. HR-TEM analysis o he PPy NPs (Fig. 3a) showed amo phous sphe ical pa icles wi h sizes be ween 35 and 55 nm (Fig. 3d). Simila ly, HR-TEM analysis o he Au/PPy NPs (Fig. 3b) displayed he p esence o seg ega ed PPy and Au sphe ical pa icles; no ice he b igh ness/con- as diffe ences, which indica e he p esence o bo h, Au (highe con- as ) and PPy (low con as ) NPs. The numbe o PPy NPs we e e- ma kably la ge in compa ison o he Au NPs in he es ed sample, confi ming he ela i ely low amoun o Au NPs in he Au/PPy mix u e. The a e age pa icle size analysis indica es ha he mean diame e o Au NPs is app oxima ely 15 nm o a size dis ibu ion anging be ween 12 and 25 nm (Fig. 3e). HR-TEM o he Au NPs emphasized hei high c ys allini y showing la ice inge spacing o 0.24 nm (Fig. 3c). This is consis en wi h he (111) plane o he ace cen ed cubic ( cc) gold (d= 2.35500 Å, ICCD ca d no. 04-0784). In summa y, he analysis o he solu ions p epa ed o spin coa ing o he second guiding/sensi i e laye s demons a e he syn hesis o bo h PPy and Au NPs. UV– is and HR-TEM poin ed ou o weak chemical in e ac ions be ween he Au and he PPy NPs in he mixed solu ions. 3.2. Cha ac e iza ion o PPy and Au/PPy NPs in eg a ed wi h Lo e-wa e pla o ms SEM (Fig. 4a) and AFM (Fig. 4b) analysis o he second guiding/ sensi i e laye s, a e spin coa ing o he L-SAW ansducing pla o ms, displayed uni o m films wi h g anula mo phology. The films p o ed good adhesion o he subs a e and he hickness o he films obse ed by c oss-sec ional SEM images (see Fig. 1b) was ound o be 350 ± 50 nm. Fo his hickness o he guiding laye (PPy, Au/PPy), L- SAW senso s showed he app op ia e a enua ion (low inse ion loss). The PPy and Au/PPy guiding/sensi i e laye s analyzed by scanning p obe mic oscope (Fig. 4c) in an a ea o 2 μmx2μm showed ela i ely low RMS ( oo mean squa e) oughness o app oxima ely 2.5 ± 0.4 nm. The uni o m and op imal hickness, low su ace ough- ness, and s ong adhesion o he film o he subs a es a e significan ac o s ha a o he a enua ion o he acous ic wa e p opaga ion and in u n he senso sensi i i y [2b]. In o de o confi m he inco po a ion o he Au NPs in o he spin coa ed guiding/sensi i e laye s, he samples (PPy, Au/PPy (1:10) and (1:2)) we e in es iga ed by TOF-SIMS echnique. SIMS is a semi- Fig. 7. F equency shi s eco ded on he non-modified PPy and modified PPy L- SAW senso s wi h low (1:10) and high (1:2) gold loadings a 2, 5 and 10 ppm o (a) ammonia and (b) e hylene. M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 5 quan i a i e me hod, which allows o ob ain in o ma ion o solids by spu e ing he solid su ace and collec ing ejec ed seconda y ions om he opmos su ace. Fig. 5a shows he compa a i e SIMS spec a o Au − molecula ion wi hin he samples. These esul s demons a e he p esence o cha ac e is ic agmen ed ions o Au - a 196.97 m/z (ma ked a ea) in he Au/PPy samples, in con as o he ba e PPy samples. Mo eo e , he in ensi y o he coun s o Au − ions is consis en wi h he amoun o Au NPs used o each sample; no ice he sha pe and mo e in ense Au - peak o Au/PPy (1:2) compa ed o Au/PPy (1:10). The co ela ion o he Au - ion in ensi y (collec ed in he dep h p ofile o 200 nm) and Au NPs loadings in he samples (Fig. 5b) confi med he p opo ional inc ease o Au - signal wi h espec o he Au NPs amoun s in oduced o he samples. Addi ionally, he p esence o cha ac e is ic agmen s o PPy (e.g., C x H y N - ), PVA (e.g., C x H y O z ) and FeCl 3 (e.g., FeClN - , FeCN - ) we e confi med in he dep h p ofile o samples by SIMS analysis. Fig. 6 shows he cha ac e is ic signals ob ained by measu ing he ansmission sca e ing pa ame e (S 21 ) o he L-SAW senso s be o e (only SiO 2 guiding laye ) and a e in eg a ion o second guiding/sen- si i e laye (PPy). Ve y simila elec ical esponse has been obse ed o all es ed senso s (PPy, Au/PPy (1:10) and (1:2)), wi h he equency and he inse ion loss o 161.6 ± 0.4 MHz and 22.4 ± 0.5 dB, e- spec i ely. The dec ease in he equencies o he PPy o Au/PPy L- SAW senso s wi h espec o he e e ence (165.2 Hz and -18.2 dB) is consis en wi h he inc ease o weigh on he piezoelec ic subs a e (mass loading effec ). Addi ionally, iscoelas ic p ope ies o PPy also can con ibu e o equency changes and lead o changes in de ice a - enua ion. In summa y, he inco po a ion o he second guiding/sensi i e laye s wi h he piezoelec ic subs a es demons a es simila mo pho- logical and elec ical p ope ies o he non-modified and gold modified PPy laye s, despi e he wo diffe en gold loadings inco po a ed in he modified laye s. 3.3. L-SAW gas sensing es s The ammonia and e hylene gas sensing es s o he L-SAW senso s based on PPy and Au-modified PPy NPs (1:10 and 1:2) a oom em- pe a u e a e p esen ed in Fig. 7. In gene al, all senso s egis e ed po- si i e esponses upon exposu e o diffe en concen a ions o a ge analy es. The senso s based on Au/PPy NPs demons a ed enhanced sensing p ope ies owa ds bo h es ed gases compa ed o he senso s based on non-modified PPy NPs. We obse ed u he ha he senso s based on low Au loadings (i.e., 1:10) p o ided be e esponses o ammonia (Fig. 7a) compa ed o hose based on high Au loadings (i.e., 1:2). Namely, Au/PPy (1:10) senso s showed highe equency shi s and an inc ease o he esponse by app oxima ely 5, 4 and 3 imes o 2, 5 and 10 ppm o ammonia, espec i ely, as compa ed o PPy senso s. In con as , he esul s ob ained o e hylene (Fig. 7b) egis e ed imp o ed esponses o Au/PPy (1:2) senso (highe Au loading) compa ed o Au/ PPy (1:10) senso , as opposi e o ha obse ed o he ammonia es s. The equency shi s o Au/PPy (1:2) senso s wi h ega d o non-mod- ified PPy senso s is inc eased app oxima ely by 4 imes o 2, 5 and 10 ppm o e hylene. The enhanced esponse o he Au/PPy (1:2) senso s o e hylene could be ela ed o he highe loading o Au NPs in his sample and he p o ed ca aly ic ac i i y o gold, which encou ages he oxida ion o e hylene a oom empe a u e [23]. Fig. 8a and b show he esponse o Au/PPy (1:10) and Au/PPy (1:2) L-SAW senso s o a ious concen a ions o ammonia and e hylene, espec i ely, wi h he equency changes o he senso s showing p o- po ional inc emen o gas concen a ion. The dec ease o equency (nega i e equency shi ) ob ained du ing he exposu e o he L-SAW senso s o ammonia and e hylene a e mos likely connec ed o he dominance o mass loading effec s, a he han he elec ical effec s, Fig. 8. (a) Response o he Au/PPy (1:10) based L-SAW senso o 2, 5 and 10 ppm o ammonia. (b) Response o he Au/PPy (1:2) based L-SAW senso o 2, 5 and 10 ppm o e hylene. Time-dependen esponse cu e o (c) Au/PPy (1:10) senso o 5 ppm o ammonia and (d) Au/PPy (1:2) senso o 5 ppm o e hylene. M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 6 which a e expec ed o cause posi i e equency shi du ing he in e - ac ion o PPy and hese educing analy es [24]. This ac is connec ed wi h he low conduc i i y (elec ical esis ance be ween 500 MΩ–1 GΩ) o he second guiding/sensi i e laye and he insignifican e- sis ance changes eco ded du ing i s exposu e o gases such as ammonia and e hylene ( his was p o ed by es ing he PPy and Au/PPy laye s deposi ed di ec ly on he op o he elec odes o bo h a ge analy es). Simila ly, he elas ic effec s in he esponse o he L-SAW senso s can also be dis ega ded, as gene ally he elas ic changes in he sensi i e ma e ial a e associa ed wi h a equency inc ease (a posi i e equency shi ) [24b]. The e o e, he nega i e equency shi s eco ded on he L- SAW senso s a e associa ed o he changes in he p opaga ion pa h o he su ace wa es induced by he mass loading o he analy es a he sensi i e laye ; whe e he mass loading e e s o physical/chemical so p ion o a ge gas molecules a /in he sensi i e laye . Fig. 8c and d show a de ailed iew o he esponse and he ime equi ed o each 90 % o he esponse and eco e y. In gene al, he non-modified and gold-modified L-SAW senso s showed simila e- sponse and eco e y imes o he same analy e and concen a ion. The esponse and eco e y imes owa d e hylene we e longe han o ammonia. Fo ins ance, esul s show ha he esponse and eco e y imes we e 59 s and 72 s o Au/PPy (1:10) senso s o 5 ppm o am- monia, espec i ely, and 81 s and 142 s o Au/PPy (1:2) senso s o 5 ppm o e hylene. This may be ela ed o he diffe ence in molecula size o hese gases (kine ic diame e o e hylene molecule o 0.42 nm [25] and ammonia molecule 0.36 nm [26]), which slows down he diffusion o e hylene molecules wi hin he sensi i e laye espec o ammonia [26], making he esponse and eco e y ime longe o e hylene. Fig. 9a shows he sensi i i y o PPy, Au/PPy (1:10) and Au/PPy (1:2) L-SAW senso s o ammonia and e hylene o concen a ion be- ween 2 ppm and 10 ppm. The sensi i i y is defined as he a io be ween he change in senso s esponse (Δ equency shi ) and a fixed analy e concen a ion change (ΔC). The sensi i i y o he Au/PPy (1:10) senso s o ammonia and e hylene was ound o be ∼1.8 imes highe han ha o non-modified PPy senso s. The sensi i i y o he Au/PPy (1:2) sen- so s wi h espec o he PPy senso s was imp o ed ∼2.2 and ∼3.2 imes o ammonia and e hylene, espec i ely. The sco e plo ob ained by p incipal componen analysis (PCA) o he non-modified and Au- modified PPy senso s (Fig. 9b) shows an app aisal o he selec i i y and he possibili y o disc imina e ammonia and e hylene by concen a ions using and a ay o he non-modified and gold-modified L-SAW senso s. Table 1 compa es he key cha ac e is ics and esul s o ou L-SAW senso s wi h o he piezoelec ic senso s epo ed in he li e a u e. The esul s show ha he senso s desc ibed in his wo k possess highe equency shi s (i.e., esponse) o he lowes es ed analy e con- cen a ion (2 ppm) han o he piezoelec ic de ices such as Rayleigh and Shea Ho izon al SAW o BAW (e.g., qua z c ys al mic obalance) senso s. The enhanced sensi i i y o ou L-SAW senso s, compa ed o o he ype o SAW senso s, is a ibu ed in pa o he mul iguiding laye s uc u e o hese senso s, which combines he p ope ies o he SiO 2 and he gas sensi i e PPy laye s o ap he wa e ene gy nea he su ace. This s uc u e slows down he wa e p opaga ion eloci y and makes he L-SAW senso s sensi i e owa ds any changes occu ing on he su ace [2a,27]. Addi ionally, he imp o ed ammonia and e hylene sensing pe o mances o Au/PPy senso s, wi h espec o he non- modified PPy, can be a ibu ed o he ca aly ic effec o he Au NPs and he subsequen en ichmen o PPy su ace by spill-o e o eac i e species h ough ca alysis [28]. The influences o he PPy modifica ion by diffe en Au NPs loadings on he L-SAW senso esponses o am- monia and e hylene a e sligh ly diffe en . The highe densi y o Au NPs on PPy L-SAW senso s (Au/PPy (1:2)) a o s he sensi i i y o e hylene. P e iously was desc ibed ha he oxida ion o e hylene is inc eased by he inc ease o Au loadings a oom empe a u e [23] in con as o Fig. 9. (a) Sensi i i y (Hz/ppm) o he L-SAW senso based on PPy NPs and Au- modified PPy films (Au/PPy 1:10 and Au/PPy 1:2) o ammonia and e hylene (calcula ed o concen a ions be ween 2 ppm and 10 ppm). (b) Sco es o p o- jec ions o measu emen s in an o hogonal base o p incipal componen s ana- lysed o he PPy, Au/PPy (1:10) and (1:2) L-SAW senso s. Table 1 The compa ison o sensing cha ac e is ics o ammonia and e hylene gas senso s based on diffe en piezoelec ic ansducing pla o ms. Type o senso S uc u e Sensi i e laye Gas RH (%) Temp. (ºC) Response (Hz) LTC (ppm) LOD (ppm) Re . L-SAW ST-Qua z/SiO 2 Au/PPy Ammonia 0 24 898 2 0.067 This wo k L-SAW ST-Qua z/SiO 2 Fe 2 O 3 /WO 3 Ammonia NR NR 627 25 1 [30] R-SAW ST-Qua z PPy Ammonia NR NR 20 10 NR [7b] SH-SAW ST -Qua z ZnO Ammonia 25 25 110 10 NR [31] QCM AT- Qua z c ys al PDA/HMSSs Ammonia 50 25 250 10 NR [32] L-SAW ST-Qua z/SiO 2 Au/PPy E hylene 0 24 690 2 0.087 This wo k R-SAW XY LiNbO 3 ZnO E hylene NR NR 200 100 NR [33] QCM AT- PC AgBF 4 /PVP E hylene NR NR 115 1 0.42 [34] RH: Rela i e humidi y, Temp.: Tempe a u e, LTC: Lowes es ed concen a ion, LOD: Limi o de ec ion, Response: F equency shi , L-SAW: Lo e wa e SAW; R-SAW: Rayleigh SAW; SH-SAW: shea ho izon al SAW; ST, XY, AT: e e o c ys al cu ; QCM: qua z c ys al mic obalance; PC: piezoelec ic c ys al; PDA/HMSSs: poly- dopamine/hollow mesopo ous silica sphe es; AgBF 4 /PVP: AgBF 4 / poly inylpy olidone, NR: Non- epo ed. M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 7 ammonia, which equi ed a empe a u e ac i a ion [29]. The high esponses o ou L-SAW senso s also con ibu e o lowe LODs han hose o o he de ices. Fo ins ance, he LOD o ammonia was 67 ppb using Au/PPy (1:10) and o e hylene 87 ppb using Au/PPy (1:2) senso s, conside ing ha he minimum signal in ensi y is 3 imes highe han noise. These LODs a e wi hin he limi s equi ed in he ea ly diagnosis o OS dependan diseases (e.g. schizoph enia, diabe es). Acco ding o he li e a u e, he concen a ions o gaseous bioma ke s ound in heal hy con ols and pa ien s wi h OS dependen diseases os- cilla e om 8 o 246 ppb o e hylene and om 290 o 2020 ppb o ammonia [14b,35]. Fu he es s o he senso s o e hylene and am- monia in humid ambien egis e ed a loss o he esponse o app oxi- ma ely 2 imes a 10 % RH and 5 imes a 30 % RH. Addi ionally, he humidi y in oduced ins abili y o he signal, making he esponses less ep oducible and eliable, pa icula ly a he low es ed concen a ion (2 ppm). Thus, he sensi i i y o he senso s was also affec ed. Fo in- s ance, a 30% RH he sensi i i y o he Au/PPy (1:10) senso s o am- monia dec eased om 92 in d y ai o 40. Simila ly, he sensi i i y o he Au/PPy (1:2) senso s o e hylene dec eased om 270 in d y ai o 72. These significan changes in he esponse o he senso s in humid ambien may be connec ed wi h he swelling o he polyme ic guiding/ sensi i e laye by wa e apo . In humid ambien , wa e molecules fill he ee olume in he polyme ic laye , making i una ailable o he gas so p ion and, hus, less esponsi e o he a ge analy e [36]. A compa ison o hese esul s and hose o he li e a u e is a he complex as mos o he epo s (e.g., hose lis ed in Table 1) do no make e- e ence o he humidi y le els du ing he es s and/o he deg ee o humidi y in e e ence in senso esponse. Howe e , based on he e- po s in Table 1 ( ow 4 and 5) o 25% and 50% RH one can no ice ha he lowes measu ed concen a ions o ammonia in d y and humid condi ions a e ye lowe o ou senso s. Humidi y in e e ence is an undesi ed and ecu en effec a RT ope a ion ha needs he im- plemen a ion o addi ional s a egies o be compensa ed. These s a e- gies may include u he uning o he polyme p ope ies using o he addi i es (hyd ophobic ino ganic ma e ials o o ganic molecules) [37] o coupling fil e s o p econdi ioning elemen s o he senso s as o e- cas ed o nex gene a ion o po able b ea h analyse [20]. In summa y, he Au-modified PPy L-SAW senso s demons a e en- hanced sensing esponses compa ed o he non-modified PPy senso s in d y condi ions, wi h be e sensi i i y and lowe LOD o e hylene and ammonia han hose epo ed ea lie in he li e a u e o o he SAW- based gas sensing elemen s a RT. The es s in humid ambien affec he senso pe o mance modi ying he LOD o highe concen a ion le els han hose ob ained in d y ambien , hus, indica ing he need o u he s a egies o compensa e he in e e ence o humidi y on he senso esponse. 4. Conclusion Lo e wa e gas senso s based on a mul iguiding laye s uc u e consis ing o SiO 2 and ei he non-modified o gold modified polypy ole we e de eloped in his wo k. Resul s demons a ed he inco po a ion o dispe sed c ys alline gold nanopa icles in o he modified polypy ole based senso s and he enhanced pe o mance o hese senso s, com- pa ed o he non-modified polypy ole based senso s, o e hylene and ammonia. Gas sensing es s in d y condi ions showed ha he de ec ion limi o he gold modified senso s eaches lowe ppb le el o bo h es ed gases (67 ppb o ammonia and 87 ppb o e hylene) as compa ed o o he simila sys ems in he li e a u e. These imp o emen s a e a - ibu ed in pa o he mul iguiding s uc u e and o he inco po a ion o gold nanopa icles in o he gas sensi i e polypy ole laye , which po en ially p omo es he so p ion o eac i e species h ough ca alysis. The inco po a ion o humidi y o he es s changed significan ly he esponse magni udes o he gold modified and non-modified senso s and in u n he sensi i i y, which dec eased app oxima ely 2 imes o he Au/PPy (1:10) senso s o ammonia and 4 imes o he Au/PPy (1:2) o e hylene. Gene ally, hese esul s sugges he need o u he im- p o emen s a he ma e ial and/o fil e ing le el o es ic he hu- midi y effec in he senso esponse. Despi e hese weaknesses, he e- la i ely easy and scalable ab ica ion o hese senso s, as well as hei sensing pe o mance owa ds low e hylene and ammonia concen a- ions make he modified s uc u es a ac i e o u u e s udies and applica ions whe e gases and/o VOCs a e in ol ed. Decla a ion o Compe ing In e es The au ho s decla e ha hey ha e no known compe ing financial in e es s o pe sonal ela ionships ha could ha e appea ed o influ- ence he wo k epo ed in his pape . The au ho s decla e he ollowing financial in e es s/pe sonal e- la ionships which may be conside ed as po en ial compe ing in e es s. Acknowledgmen s This wo k has been suppo ed in pa by he Czech Science Founda ion (GAČR) ia G an no. 17-16531S, he Spanish Minis y o Economy and Compe i i eness ia p ojec s TEC2015-74329-JIN-(AEI/ FEDER,EU), and TEC2016-79898-C6-1-R (AEI/FEDER, EU) and by Uni e sidad Nacional Au ónoma de México ia G an DGAPA-UNAM- PAPIIT TA100118. The suppo o he In e nal G an Agency (IGA) s uden junio p ojec no. STI-J-18-5559 and he Ramón y Cajal a e also acknowledged. This esea ch has made use o he in as uc u es he Spanish ICTS Ne wo k MICRONANOFABS, pa ially suppo ed by MINECO, and CEITEC Nano Resea ch In as uc u e, suppo ed by MEYS (2016-2019). We also acknowledge he suppo ecei ed om Ondřej Chmela o Scanning P obe Mic oscope analysis and om Selena Islas in he UV– is Spec opho ome e . Re e ences [1] a) A. B yan , D.L. Lee, J.F. 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He esea ch in e es s include syn hesis and cha ac e iza ion o he gas sensi i e polyme s and de el- opmen o gas senso s de ices. F. Bahos ecei ed he B.Sc deg ee in Enginee ing Physics in 2011 om he Cauca Uni e si y. He ecei ed his MEng. deg ee in Elec ical Enginee ing in 2018 and wo ked on gas senso s and RF signal condi ioning a Ins i u o de Ciencias Aplicadas y Tecnología o he Uni e sidad Nacional Au ónoma de México (ICAT –UNAM). Now, he is wo king a Sis ema de In es igación, Desa ollo Tecnológico e Inno ación (Senno a –SENA) on elec ical senso s and in e ne o hings. D. Ma a agui is g adua ed in physics in 2007, ecei ed his M.Sc. in Ad anced Ma e ials and Nano echnology in 2008 and his Ph.D. deg ee in physics in 2012 om he Uni e sidad Au ónoma de Mad id. F om 2008 o 2013 he was wo king in he GRIDSEN g oup a he CSIC (Spain) on I + D + i o bio-chemical mic osenso s and elec onic noses o en i onmen al p o ec ion. He was a Resea ch P o esso a he Na ional Au onomous Uni e si y o Mexico (UNAM), in he Biomedical De ices G oup a he CCADET on he de elopmen o chemical senso s and biosenso s. Cu en ly, he is wo king a he Ins i u o de Tecnologías Físicas y de la In o mación (CSIC) de eloping new senso echnologies. M. Po oček ecei ed his PhD in Physics in 2010. He is cu en ly wo king as esea che a he Cen al Eu opean Ins i u e o Technology CEITEC and as assis an p o esso a he Ins i u e o Physical Enginee ing, Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, B no, Czech Republic. His esea ch is ocused on su ace analysis o ul a- hin laye s su ace using Time-o -ligh Seconda y Ion Mass Spec ome y (TOF-SIMS) and The mal deso p ion spec oscopy (TDS). Z. K al ecei ed his PhD deg ee in Elec onic Enginee ing om he Uni e si y Ro i a i Vi gili, Spain, wo king on cha ac e iza ion o no el op oelec onic s uc u es like pho- onic c ys als. Sho ly a e academic ca ee he s a ed in a p i a e echnology company ocused on de elopmen and ab ica ion o scanning elec on mic oscopes. Cu en ly, he is wo king o The mo Fishe Scien ific as a senio applica ions scien is expe ienced in analy ical ins umen s and ailu e analysis echniques. J. D bohla o a ecei ed he PhD deg ee in physical chemis y in 2008. She wo ks as senio esea che and associa e p o esso a B no Uni e si y o Technology in he field o syn hesis and cha ac e iza ion o nanoma e ials o sensing in medical and en i onmen al applica ions. F om 2017 she also wo ks as Seconded Na ional Expe in he Eu opean Commission o nanoma e ials s anda diza ion, egula ion and go e nance. I. G acia ecei ed he PhD deg ee in physics in 1993 om he Au onomous Uni e si y 18 o Ba celona, Spain, wo king on chemical senso s. She joined he Na ional Mic oelec onics Cen e (CNM) wo king on pho oli hog aphy, cu en ly she is ull ime 20 senio esea che in he Mic o-Nano Sys ems depa men o he CNM and he wo k is ocused on gas sensing echnologies and MEMS eliabili y. S. Vallejos ecei ed he PhD deg ee in Elec onic Enginee ing om he Uni e si a Ro i a i Vi gili, Spain. Cu en ly, she holds a Ramón y Cajal ellowship and he esea ch is ocused on gas sensing echnologies and nanoma e ials. She is in e es ed in explo ing scalable syn hesis me hods o ailo and enginee he sensing p ope ies o nanoma e ials, as well as in he de elopmen o gas sensing mic osys ems o applica ions in sa e y, medicine, and en i onmen . M. Še ka, e al. Senso s & Ac ua o s: B. Chemical 304 (2020) 127337 9