358  Advances in Production Engineering & Management ISSN 1854 ‐6250 Volu me 15 | Number 3 | Se p te mber 2020 | pp 358–368 Journal ho me: a p em‐journal.or g https://doi.org /10.14743/apem2020.3.371 Original s cientif i c paper     Testing of novel nano gold ink for inkjet printing  Rudolf, R. a,b,* , Majerič, P. a,b , Golub, D. a , Tiyyagura, H.R. a   a University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia  b Zlatarna Celje d.o.o., Celje, Slovenia      A B S T R A C T   A R T I C L E   I N F O Gold n a n oparti cles ( GNPs) w ere synthesised b y the U ltra sonic Spray P y r o l y s i s (USP) pro ce ss a n d co llected in d eionised w ater w ith the a ddition o f a s t a b i l i s e r , i . e . PVP (0.1 w t.%). W ith the u se o f a rotary e va porator, a h ighl y c oncentrated GNPs’ s u s p e n s i o n w a s a c h i e v e d ( 6 0 0 p p m c o n c e n t r a t i o n o f G N P s ) , w h i c h was u s ed d i‐ r e c t l y a s n o v e l n a n o g o l d i n k f o r i n k j e t p r i n t i n g . T h e p h y s i c a l a n d c h e m i c a l c h a r a c ‐ teristics of s uch prepare d n ano gold i nk were explained in d e t a il b y the use of Z eta (  ) P o tent ial, A TR‐FTIR spectrosco py , UV /V I S s pectro sco p y , and n anoparticl e s ize was identified through S EM. With nano g old ink the c hosen patte rn w a s p rinted onto p hot o p aper, which was char acterised for confirming the p r esence o f gold with op t i cal an d S E M/EDX obs e rvations . The obs e rvations r e v eale d that the t ested printed nano g old ink on the p aper p rovided a new route for the f a b r i c a t i o n o f p a p e r‐bas e d el ectrochemical immun o s e ns ors , c olorime t ric s e ns ors a n d n a n o ‐ metallic bio medical s e ns ors . © 2020 CPE, Uni versity of M a r ib or. All rights re s erve d.   Keywords: Inkjet printing; Nano gold ink; Gold nanoparticles; Characterisation; Paper‐ba sed se nsor *Corresponding author: rebeka.rudolf@um.si (Rudolf, R.) Article history: Received 7 Aug u st 2020 Revised 9 Se p tember 2020 Accepted 14 Se ptember 2020      1. Introduction   Metallic nanoparticles (MNP) have c ompletely different properties like e lectrical, m echanical, optical and magnetic a s their identic a l bulk m aterials [ 1‐3]. Ba s e d o n t h i s , t h e y c a n b e u s e d f o r various t y pes of a pplicat ions, such a s in the m edical a nd e lect ronic industries [ 4‐6], as w ell as increasingl y in t he f ood i ndustry [7]. T he p rep a ration of t heir i nks is e nvi s aged a s on e of t h e growing app l ications o f MNP [8]. T he m etallic nanoparticle i nk can be p repared as v ar ious types of ink, namely, single e lement, alloy metallic, m etallic oxide and core s hell bimetallic nanoparti‐ cle i nks [9]. T hese m etallic inks, available on the present mar ket, a re prepared from Silver, C op‐ p e r , G o l d , A l u m i n i u m , C o b a l t , Z i nc, Palladium, Nickel a nd Plati num. N owadays, m an y conven‐ tional metho d s are being used f or the p rinting of M NP i nks on d ifferent s u b strates for differe nt a p p l i c a t i o n s [ 1 0 , 1 1 ] . I n k j e t p r i n t i n g i s , n a m e l y , a r a p i d l y e v o l v ing tech nol ogy o f l oadi ng f u n c‐ tional inks o nto various substrates, and also o n e o f th e excelle n t a p p r o a c h e s f o r t h e p r i n t i n g o f M N P i n k s , a s i t d e p o s i t s t h e r e q u i r e d p a t t e r n s o n t h e s u r f a c e w ith accur a te a nd non ‐contac t writing [12‐14]. The described in kjet p rinting method i s o p erate d a t r o o m t e m p e r a t u r e a n d pressure [ 8], controlled by d ata from images or a p attern in c on j u n c t i o n w i t h a c o m p u t e r , a n d transferring the i nk i n the form o f microdroplets onto the chose n s u b s t r a t e . I n k j e t p r i n t i n g h a s high p recision, a s well as c ontro l of h omogeneous m icrodroplets’ l oading, and the main a d‐ v a n ta g e i s t ha t w e c a n p r in t c o m p le x s c h e m e s w i t ho u t c h e m i c a l wa s t e a f t e r th e p r i n t i ng pr o c e s s , which makes the proc ess environ m entally friendly and economical [1 5 ]. In t h e l ast decade, inks c omposed of g old n a nop articles (GNPs) – so‐called nano g old inks – had a lot of a ttenti on towards printed electronics d ue to th e G NPs’ p r o p e r t i e s , l i k e h i g h t h e r m a l conductivity, exc e llent r esistance to o xidation a nd tuneable op tical prop erties [ 1 6 ]. Based o n Testing of novel nano gold ink for inkjet printing   Advances in Production Engineering & Management 15(3) 2020  359 this, nano gold i nks play a n import ant role i n t h e fabricati o n of printed e lectronics, biosensing applications ( e.g. m icroel ectrod e arr a ys) and elect r ochemical s ensors [ 17]. T he recent literature shows that t he p reparati on of n ano gold i nks and ink formulation s w e r e a p p l i e d b y d i f f e r e n t a p p r o a c h e s [ 1 8 ‐ 2 1 ] , b u t t h e s e a p p r o a c h e s a r e n o t b e n e f i c i a l f o r t h e p r o d u c t i o n o f l a r g e r q u a n t i ‐ ties o f nano gold i nk. Th e currently a vailabl e n an o gold i n k s o n the market a re l imited, and cost i n e f f e c t i v e w h e n c o m p a r e d t o o t h e r M N P i n k s . T h e a i m o f t h i s r e search w ork was the prepara‐ tion of n o v el n ano gold i nk f or t he i nkj e t printing p rocess. T h e synthesised gold i nk p resented i n this work is cost‐effective for the fabr ication of p aper‐based sensor applicat ions. The rest o f this a rticle i s arranged a s follows: Sec tion 2 desc ribes the synthesis and inkjet p r i n t i n g o f G N P s , w i t h t h e c a r r i e d o u t c o m p r e h e n s i v e c h a r a c t e r i sation o f n ano gold i n k, includ‐ i n g p h y s i c a l a n d c h e m i c a l p r o p e r t i e s , b y u s i n g d i f f e r e n t t e c h n i q u e s l i k e Z e t a ( ) P o te n t ia l me a s ‐ u r e m e n t s , A T R ‐ F T I R , U V ‐ V I S s p e c t r o s c o p y , a n d S E M / E D X o b s e r v a t i o ns. Section 3 discusses the r e s u l t s o f p r e l i m i n a r y t e s t i n g o f p r i n t i n g t h e p a t t e r n s b y u s i n g a Fujif i lm D im a t ix D MP‐2 83 1 inkjet p rint er. The pri n ted patt erns w ere ch aracterised through O p t i c a l M i c r o s c o p y a n d SEM/EDX ob servations. The conclusions and futur e perspectives a re presented in s e ction 4. 2. Materials, methods and design of experiments  2.1 Materials  Synthesis of GNPs GNPs w ere synthesised with a n Ultrasonic Spray Pyrolysis (USP) d e v i c e l o c a t e d i n Z l a t a r n a C e l j e d . o . o . ( S l o v e n i a ) , F i g . 1 . T h e U S P i s c o m p o s e d o f a n u l t r a s o n i c g ener ator ( with f = 2 , 4 M H z ) , a reactor furnace (with 3 temper ature zones) a nd a s ystem for nan oparticle c ollection (3 c ollect‐ ing bottles). I n th e USP, w ith the ultr asound, the precursor sol u t i o n w i t h t h e d i s s o l v e d m a t e r i a l is d ispersed i nto droplets. These drop lets a re then transported w i t h a c a r r i e r g a s t o a h i g h t e m ‐ perature r eactor, where the target m aterial inside the d roplet is d ecomposed chemically v ia p y‐ rolysis, a nd n anoparticles of pur e ele m ents a re f o r med. I n this s tudy, the s e lected r aw m at erial for prepari n g Au‐prec u rsor s olutions w as A u a cet a te s alt (AuAc, g old (II I ) acetat e (Au(CH 3 COO) 3 ), A lfa Aesar), which was dissolv ed i n deionised water and hydr ochloric a cid (HCl, 3 7 % , S i g m a A l d r i c h ) w i t h a f i n a l c o n c e n t r a t i o n o f A u 1 g / L [ 2 2 ‐ 2 6 ] . T h e p H v a l u e o f t h e p r e ‐ pared soluti on was ab ou t 1‐2. T he hi g h acidity of this solution m ay b e u n favourable f or s ome of the USP elements d urin g synthesis, a s observed f rom previo us p r actical experience. In o rder to i n c r e a s e t he p H v a l u e t o 5 ‐ 6 , t h e s o l ut i o n w a s s t i rr e d m a g ne t i c a l l y a n d pe l l e t s o f s o d i um h y d r ox ‐ ide (NaOH, F isher Che m icals) w ere added, o btaini ng a clear y e l l ow s olution , suitable f or u se w ith U S P . T h e f o l l o w i n g p a r a m e t e r s w e r e u s e d i n t h e U S P s y n t h e s i s : T he f low of c arrier g as N 2 w a s 4 L/min, t he f l o w of r eduction gas H 2 w a s 2 L / m i n , r e a c t o r t e m p e r a t u r e s w e r e T 1 = 1 2 0 ° C , T 2 = 40 0 °C, T 3 = 4 0 0 ° C , t h e c o l l e c t i o n m e d i u m w a s e t h a n o l w i t h s t a b i l i s e r P o lyvinylpyrrolidone ( P V P ) w i t h a c o n c e n t r a t i o n o f 2 . 5 g / L . W i t h U S P t h e f o r m e d G N P s w ere collected i n the de‐ scribed collection system. Preparation of nano gold ink The nano gold i nk w as p repared directly f rom GNPs e thanol/ PVP s uspens ion which was, a fter USP synthes i s completion, immediat ely subjected to a c oncentrat ion process by u sing a rotary evap orator. The p a ramet e rs o f th e ro tary e vapor a tion process we re : Ro ta tio n : 2 40 rpm , P re s‐ sure: 40 m Bar, Bath temperature: 4 0 °C, Initial vo lume: 2 50 m L , D i s t i l l a t i o n t i m e : 4 5 m i n , F i n a l volume: 5 mL. The concentration o f G NPs in s uspension was measur e d w i t h I n d u c t i v e l y C o u p l e d Plasma‐ M ass Spectromet r y (ICP‐MS). The spectromet er u se d was an H P , A g i l e n t 7 5 0 0 C E , e q u i p p e d w i t h a c o l l i s i o n c e l l ( S anta C lara, CA, USA). T h e foll owing co nditions f or I CP‐ M S were used: Th e po wer was 1.5 kW, Nebu liser‐Meinhard, plasma g as flow w a s 1 5 L / m i n , n e b u l i s e r g a s flow w as 0 .8 5, m ake up g as f low w a s 0.28 L /mi n , and re acti on gas f l o w w a s 4 . 0 m L / m i n . T h e instrument w as c alibrat e d with m atr i x m a tched c a libration so lut ions. The relative m easurement uncertainty was estimated as ±3 %. T he c o n cent ration o f G NPs in h ighly concentrat ed s uspen‐ sion as novel nano go ld in k was 6 0 0 p pm. Rudolf, Majerič, Golub, Tiyyagura    360  Advances in Production Engineering & Management 15(3) 2020 Fig. 1 Ultrasonic Spray Pyrolysis (USP) device located in Zlata rna Celje d.o.o. Inkjet printing of nano gold ink The Inkjet p rinter u sed for the printing o f n a no gold i nk, was the Fujifilm D imatix D MP‐2831 ( F U J I F I L M D i m a t i x , U S A ) w i t h a p i e z o e l e c t r i c I n k j e t c a r t r i d g e . The printing o f patterns o n a sub‐ s t r a t e u s i n g p r e p a r e d n a n o g o l d i n k w a s u n d e r t h e f o l l o w i n g p a r amet ers: W ave f o r m: low‐ viscosity (th e u ni for m ity of the d roplets was th e best); J ettin g V olt a ge f or a ll noz zles w as 1 4 V; Tickle c ontr o l was 23 kHz; H ead Angle was 9.5° ; Cartridge tempe r a ture w as 23.5 °C a nd t he Printing p lat e t emper a ture w as 24.5 °C. The print pattern w as c omposed of v ertical lines with a thickness of 3 mm and a length o f 60 mm. T h e s elected patter n w as p rinted on a commercially o b t a i n e d g l o s s y p h o t o p a p e r . T h e p a t t e r n w a s p r i n t e d i n 1 5 l a y e rs in order to e nsure a more continuo us distribution o f GNPs fr om the n ano gold ink. 2.2 Characterisation of nano gold ink and GNPs  Zeta ( ) Potential measurement The Zeta (  ) Potenti a l of n ano gold i nk, using the Dynamic Light Scatterin g (DLS) technique, w as measur ed w ith the Malv ern Zet a si zer Nano ZS ( Malvern Panalytica l, U K), and a folded c apillary zeta c ell. T h e s elected m e asure m en t param e ters w ere: R e f rac t ive I n d e x ( R . I . ) f o r t h e g o l d p a r t i ‐ cles was 0.2, a bsorba nce was 3.32, d ispersant was ethan o l, tem p erature wa s 25 °C, R.I. f or e tha‐ nol was 1 .3 6, viscosity w a s 1.10 cP, die l ectric constant w as 2 2.4. ATR ‐FTIR spectroscopy A t t e n u a t e d T o t a l R e f l e c t a n c e – F ourier T ransform I nfr a red Spect roscopy (ATR‐FTIR) a nalysis of the nano g ol d ink and pu re PVP – a s control – was performed wit h a Perkin–Elmer F TIR Spec‐ trophotom e t e r and a Gol d en G ate A t t e nua t ed T ot al R eflectio n atta c h m e n t w i t h a d i a m o n d c r y s ‐ tal. T he A TR ‐FTIR spectr a were a ccumulated within 16 scans at a r esolution of 4 c m ‐1 w i t h i n a ra nge of 4 000 cm ‐1 t o 65 0 cm ‐1 . UV/VIS spectroscopy The UV/ V IS a bsorption o f n ano gold i nk w as m eas u red with a T eca n In finit e M 20 0 UV/ V IS Spec‐ trophotom e t e r (Tecan, A u stria), using a qu artz c uvett e . Th e abs orb a nce measur e men t s wer e made o v e r t h e wa vele ngth r an ge o f 30 0‐70 0 nm, with no. f la shes = 5x a n d t ime p e r measur e = 20 ms.    Testing of novel nano gold ink for inkjet printing   Advances in Production Engineering & Management 15(3) 2020  361 2.3 Optical and SEM/EDX observations  Optical microscopy The Inkjet p rinted l ayers were e xami ned on th e Nikon EPI P HOT 30 0 light microscope (Nikon, Japan). SEM microscopy/EDX analysis A Scanni ng E lectron Mic r oscope ( SEM), Sirion 400 NC (FEI, U SA) with a n En ergy‐Di s persive X‐ ray spectroscope ( EDX) I NCA 350 (Oxford Instrum e nts, U K), was u sed for the SEM investigations o f n a n o g o l d i n k a n d p r i n t e d p a t t e r n s . D r o p l e t s o f t h e n a n o g o l d ink were put on S E M holders with conduct i ve c arbo n adhesive t ape and left t o dry under vacu um, whil e the printed patterns were l ocated d irectly on the SEM h o lder with conductive carb o n adhesi ve t ape without any addi‐ tional tr eat m ent. EDX was used for the determination of qualitati v e a n d semi‐quantitative chemical composition. GNPs’ size measurements G N P s ’ s i z e m e a s u r e m e n t s i n n a n o g o l d i n k w e r e p e r f o r m e d f r o m S E M micrographs. The sizes w e r e m e a s u r e d w i t h t h e m i c r o s c o p e s o f t w a r e a n d w i t h m a n u a l m e a s urements f r o m the SEM micrographs , measured w ith the I m ageJ a n a lysis software. Th e pa rticle s ize distributions show the man ual m easur e m e n t s from the I m ag eJ sof t wa re . Th e GNP s ’ s ize dis t ributions w ere made from 1 ,000 n anop article m easureme nts f o r ea ch m ea surem e nt sam pl e. T wo t ypes o f GNPs ’ measur em en ts w ere do n e : Firstly, o n the pr epare d n an o gold i nk b e for e f iltration, a nd s econdly a f t e r m a n u a l f i l t r a t i o n o f n a n o g o l d i n k t h r o u g h t h e i n j e c t i o n filters with 0 .1 µm pore s izes. These me asureme n ts w ere mad e t o determine i f t he i nk w as a dequa t e t o prevent cl oggi ng o f GNPs in the nozzle durin g printin g. 3. Results and discussion  3.1 Zeta (  ) potential measurement  Zet a (  ) Potential measur ement is a s ignific a nt c haracterisation techn ique t o determi n e the sur‐ f a c e c h a r g e o f G N P s i n n a n o g o l d i n k s , w h e r e t h e  p otenti al c an b e emplo y ed f or u nde r standi n g the physical s tability o f G N Ps [ 27]. It i s generally c onsidered th a t the GNPs w ith high n egative or positive  p otential a re e l e ctrically s tabilised, w hil e G NPs with l ow  p otent i als tend t o coagulate [28]. There are m a ny o t h er f actors, such a s the presence o f sta bilisers, that a ffect the physical s t a b i l i t y o f n a n o g o l d i n k s . T h e m e a s u r e d  p ot en tial o f GNPs i n the studied nano gold i nk w as ‐ 1.89 mV, a n d the  p o t e n t i a l d i s t r i b u t i o n c a n b e o b s e r v e d f r o m F i g . 2 . A n e g a t i v e  p o t e n t i a l i n d i c a t e s t h a t G N P s w e r e n e g a t i v e l y c h a r g e d , a n d t h i s i s m o s t l ikely bec a use GNPs a re c apped w i t h t h e P V P s t a b i l i s e r , w h i c h l o w e r s t h e m a g n i t u d e o f  p ot ential. The G N Ps c apped with P VP stabiliser r emained dispersed in t h e n a n o g o l d i n k s m o s t l i k e l y d u e t o t h e s t e r i c h i n d r a n c e c r e ‐ ated by the l a rge PVP layer coat i n g on the sur face o f th e GNPs [ 29].    Fig. 2  po t enti al dis tributio n of gold ink    Rudolf, Majerič, Golub, Tiyyagura    362  Advances in Production Engineering & Management 15(3) 2020 3.2 ATR‐FTIR spectroscopy  ATR‐F T IR S p e ctroscopy was used f or t he d etermi nation o f P V P stab i lise d GNPs i n na no go ld ink – t h e F T I R s p e c t r a o f p u r e P V P a n d G N P s i n n a n o g o l d i n k a r e s h o w n i n F i g . 3 . T h e F T I R s p e c t r a for pure P VP s hows c haracteristi c absorption peaks for amide N‐ H stretch at 3 398 cm ‐1 , C‐N vi‐ bration at 1 2 79 cm ‐1 , C=O stretching a t 166 7 cm ‐1 a n d t y p i c a l p e a k s f o r t h e p y r r o l i d i n y l g r o u p o f PVP at 1 475 c m ‐1 a n d 1 4 3 2 c m ‐1 . T h e c h a r a c t e r i s t i c p e a k s o f p u r e P V P a l s o e x i s t e d i n P V P s t a b i‐ lised nano g old ink, indicating that th e a dsorption of PVP m ole cules onto the GNPs ’ s u r f a c e w a s successful [ 29,30]. T he s uccessful f unctionalisat ion, m ost like ly v ia i ntermolecul a r hydrogen bonding, w as a lso confirmed with a n absorption peak s hift o f C= O stretching f rom 1667 cm ‐1 t o 16 43 c m ‐1 . Fig. 3 FTIR spectra of pure PVP polyme r (black) and nano gold ink (red) 3.3 UV‐VIS spectroscopy  UV‐VIS s p e ctroscopy was used f or e valuati on o f the o ptical a nd structural p roperties of n ano gold i nk, i.e. i nvestigation o f th e int e ractions b etw een n an o g ol d ink with d iffer e nt e lectromag‐ netic waves. G NPs have u nique optical properties, along w ith a p r o p e r t y k n o w n a s S u r f a c e P l a s ‐ mon Reson a nce (SPR) [31‐32]. W it h SPR, the a bsorption of a s pec ific w av elen gth caus es t he f luc‐ tuation of e l e ctrons on t h e GNPs ’ s u r f a c e . S P R i s s t r o n g l y d e pe n d e n t o n t h e G N P s ’ s i z e , s h a p e a n d their a gglo m eration state . I t is known that G NPs d i splay a sin g le a bsorptio n peak i n th e visible range b e twe e n 51 0‐5 50 nm, a n d due to t h e G NPs ’ s i z e v a r i a t i o n s , G N P s i n k s h a v e d i f f e r e n t c o l ‐ ouration [ 3 3 ] . The absor b ance s pectr a o f the prep ared n ano gold i n k c a n b e o b s e r v e d f r o m F i g . 4 . I t w a s d i s c o v e r e d t h a t t h e m a x i m u m a b s o r p t i o n b a n d i s a t ~ 5 3 8 nm, w hich indicates the sta‐ ble stat e o f t he prep a red nano gold in k. Fig. 4 UV‐VIS spectra of nano gold ink Testing of novel nano gold ink for inkjet printing   Advances in Production Engineering & Management 15(3) 2020  363 3.4 GNPs’ size measurement  GNPs ’ s ize measur em en t was perfor med using SEM microgr aphs, firstly on the pr ep ared n ano g o l d i n k b e f o r e f i l t r a t i o n ( F i g . 5 a ) , a n d s e c o n d l y a f t e r m a n u a l f iltration of n ano gold i nk t hrough injection filters with 0.1 µm pore sizes (Fig. 5b). T h e c a l c u l a t e d m e a n G N P s ’ s i z e i n t h e n a n o g o l d i n k b e f o r e f i l t ration was 46.2 n m , with a ma ximu m m e asured p art i cle s ize of 332 nm. Th e smaller GNPs h ad a mostly s pherical s hape, while the lar g er ones had mor e irregular shapes. After filtrati o n , the me an G NPs size d ropped to 16.7 nm. Th e mi nimu m measur ed G NPs’ s ize i n t he f iltrated n ano g o l d i n k w a s 5 . 7 n m , a n d t h e maximu m p a rticle s ize was 38 nm. The GNPs ’ size m easur e ments co nfir med that the f iltration had removed larger GN P s and clust ers of a gglomerated GNPs s ucce ssfully f rom the prepared nano g old in k, m aki n g th e ink usabl e f or t h e p rint ing with out c logging o f the cartridge nozzles. Mostly s pherical a nd s ome irregular particles remained i n the na n o g o l d i n k a f t e r f i l t r a t i o n , a s observed from Fig. 5. Fig. 5 GNPs’ size distributions: a ) be fore and b ) after filtration of nano gold ink 3.5 Inkjet printing of nano gold ink  T h e p r e p a r e d n a n o g o l d i n k w a s i n j e c t e d i n t o t h e r e s e r v o i r b y m eans o f a syringe and a special metal needle. After the reservoi r was filled, a print h ead was attached to the reservoir, a nd the assembl e d cartridge was inserted i n t h e appropriate location i n t h e I n k j e t p r i n t e r . T h e c a r t r i d g ‐ es u sed for the printing w ere Dim a ti x brand (DM P D MC‐1 16 1 0 ), w it h t h e m a x i m u m r e s e r v o i r v o l u m e o f 2 m L . T h e s u i t a b l e c a r t r i d g e w a s f i r s t s e l e c t e d i n t h e computer p rogramme. The print h e a d w a s c l e a n e d b y l e a k i n g n a n o g o l d i n k t h r o u g h t h e n o z z l e s , to m ak e sure t hat no noz zles were clogged. After the print head w as cleaned, a substrate was i n s e r t e d i n t o t h e p r i n t e r a n d was adhered onto th e pri n ting p l a te a t the ed ges. T his prevent s t h e s u b s t r a t e f r o m m o v i n g d u r ‐ ing printing . Be f o re printing , the na n o g o ld ink f low throug h t he nozzles was c hecked agai n . With the pro g ram m e, th e i ndi v idual n o zzl e and all th e nozzles to g e th er w ere ch ecked (Fi g . 6 a ), to en‐ sure t hat th e n a no g old ink droplets w ere ho mo gen e ous and unifor m . T h i s w a s a c h i e v e d w i t h the modifica tion o f th e in k wav eform (Fig. 6 b ) and with cha ngi n g th e i n k dr oplet par a meters. Rudolf, Majerič, Golub, Tiyyagura    364  Advances in Production Engineering & Management 15(3) 2020 Fig. 6 a) Image of ink jetting through nozzles at the applied voltage of 14 V, which is the thres hold for proper drop for m ation and bre a ka ge using the wavefo; b) wavefor m custo m i s ed fo r thi s study   3.6 Optical microscopy   The nano go ld i nk I nkj e t printing p erformance o f vertical l in es p rinted o nto phot o p a per w a s observed b y optical micr oscope, as s hown in Fig. 7 a. I t c a n be seen that th e printed line w idths between eac h line do not c orrespond, m o st like ly due to the ove rla ying b e t we e n the sprea ding of the droplets. Th e line wid th bec omes s maller with i ncreasing in t he drop sp a c ing, which i s due to d e c r e a s i n g i n t h e o v e r l a p p i n g o f t h e n a n o g o l d i n k d r o p l e t s . T h e me asured a verage p ri nted l ine width was ~130 μm. Fig. 7 Optical m icroscope image of nan o gold ink Inkjet printing perf ormance : a) Vertical line s on photo paper; b) Macro imag e of printed pat te rn; c) Schematic representation of the patter n printing direc t ions     Testing of novel nano gold ink for inkjet printing   Advances in Production Engineering & Management 15(3) 2020  365 3.7 SEM microscopy/EDX analysis  A typical SEM micrograph of a n an o gold i nk p ri nted p att e rn on p u r e p h o t o p a p e r i s s h o w n i n Fig. 8 a. T h e g old printed pattern w as i nvestigated thoroughl y f or d eter minati on o f t he G NPs’ d i s t r i b u t i o n ( F i g . 8 b ) . T he G N P s w e r e c l u s t e r e d t o g e t he r t o a g r e a t e r e x t e n t i n t h e o p t i m a l r e g i o n ( F i g . 8 c ) . E D X a n a l y s i s c o n f i r m e d t h a t t h e p r i n t i n g w i t h n a n o g old ink was successful, as the chemical c o m position of the clusters showed m or e than 4 0.1 % of A u (Fi g . 8d). T he e lements C, O , A l a n d S i w e r e o r i g i n a t i n g f r o m t h e p r i n t i n g s u b s t r a t e – a c ommercially o btained glossy photo paper. T h e S E M a n a l y s e d p r i n t e d p a t t e r n d o e s n o t s h o w a c o n t i n u o u s l a y e r o f G N P s , b u t r a t h e r v i s i ‐ ble printing l ines, which contai n clusters of GNPs and discrete GNPs. T he se initia l re sults ide ntif y t h a t t h e p r i n t i n g p a r a m e t e r s w o u l d h a v e t o b e m o d i f i e d f o r p r i n t i n g o f a m o r e c o n t i n u o u s l a y e r o f G N P s : P r i n t n o z z l e s i z e , r h e o logical properties o f the nano gold ink (sur f ace tensio n and vis‐ cosity), G NPs’ c oncen trat ion in th e n ano gold i nk, GNPs’ sizes, d roplet s pacing, print overlapping and printing d irection [34]. There are some l imitations f or the s e p aramet er s that a lso n eed t o b e considered for printing o f a contin uou s layer. The print nozzle size, along with t he r heologic al properties of t h e n a n o g o l d i n k , d e t e r m i n e s t h e i n k f l o w f o r p r i n t i n g . T h e s e p r o p e r t i e s n e e d t o k e e p t h e n a no gold i nk i nside the noz zl e without the i n k flowin g fr eely from t h e noz zle when t he p rintin g is s topped and provide the flow o f t h e i n k a n d d r o p l e t g e n e r a t i o n w h e n t h e I n k j e t i s a c t i v a t e d . S t a b i l i s e r s f o r G N P s m a y a l t e r t h e rheologic a l properties o f the nan o g ol d ink, w hich i s something t o consider d uring nan o g old ink formul ation. Fig. 8 SEM micrograph of Inkjet printe d GNPs with corresponding EDX a nalysis: a) Pure pho to paper; b) Printed pattern with the spa ce between the lines; c) GNPs’ d is tributio n; d) EDX analys is Rudolf, Majerič, Golub, Tiyyagura    366  Advances in Production Engineering & Management 15(3) 2020 T h e a v a i l a b l e n o z z l e s i z e s a l s o l i m i t t h e G N P s ’ s i z e s i n n a n o g o l d i n k , w h i c h c a n b e u s e d f o r successful p rinting. O ur e xperimen tal results show t hat GNPs o f s i z e a r o u n d 1 0 n m a r e s t i l l c a ‐ pable of p rinting, w hile l arger sizes clog the printing n ozzl e, m a k ing na no g o ld in k un usa b le . S i n c e t h e G N P s i n n a n o g o l d i n k , o b t a i n e d w i t h t h e U S P t e c h n i q u e had a br oad size d istribution, f i l t e r i n g o f n a n o g o l d i n k w a s m a n d a t o r y f o r p r i n t i n g w i t h a n I nkjet print e r. A nother point to consider i s the conc entr ation of G NP s in the n an o gold i n k a nd their stability. U sing v ery hi gh concentrati o ns o f GNPs ( > 60 0 ppm ) leads to u nstable dispersion s (depending on the st abiliser u s e d ) , r e s u l t i n g i n a g g l o m e r a t i o n , r e n d e r i n g t h e n a n o g o l d i n k unusable. Very h igh concentra‐ tions m ay a l s o lead t o un wanted I nkj e t behaviour , i nterferin g w ith ink droplet sizes and droplet g e n e r a t i o n i n t e r v a l s . T h e d i s t r i b u t i o n o f G N P s i n s i d e t h e d r o p l e t i s a l s o a f a c t o r , a f f e c t i n g t h e printed patt ern. T hes e c ombin e d nano gold i nk p roperties may r e sult i n various forms of the printed patt erns. One su ch e xampl e i s the “co f fee‐ ring e ffect” [ 9 ] , w h e r e m o s t o f t h e G N P s w o u l d be clustered on the droplet outer diameter, while the inside o f t h e p r i n t e d d r o p l e t h a s a d i s p r o ‐ portionat e ly low nu m ber of GNPs. P o s s i b i l i t i e s f o r i m p r ov i ng t h e p r in t e d p a t t e r n m a y b e by m o d i fying the droplet spacing, p rint overlapping a nd printing directi on, i n order to p roduce m ore fav o u r a b l e r e s u l t s f o r p r i n t i n g a more c ontinuous l ayer o f GNPs. Se vera l ex pe riments n ee d to b e p erfor m ed in order t o f ind th e optimal bal a nce of n ano gold i nk p roperties, G NP s’ p roperties a nd p rinting parameter s . Here, it w a s s h o w n t h a t U S P i s c a p a b l e o f p r o v i d i n g G N P s t o b e u s e d i n I nkjet printing, while the dis‐ cussed printing specifications d epend on th e applic ation of the printed patterns. U s i n g a p r i n t e d l a y e r o f d i s c r e t e l y d i s p e r s e d G N P s o n g l a s s o r o t h e r t r a n s p a r e n t s u b s t r a t e s p r o d u c e s a s p e c t r a l l y s e l e c t i v e s u r f a c e w h i c h b l o c k s i n f r a r e d r adiation du e to the p las m on reso‐ n a n c e e f f e c t . T h i s m a k e s t h e s e p r i n t e d G N P s u s a b l e i n s p e c i a l i s ed o ptical a pplications, such a s l i g h t f i l t e r s o r l o g i c g a t e s [ 3 5 ] . A n e x a m p l e o f e x p l o i t i n g t h i s property i s also u sage i n ener gy‐ effici ent win d ows. N ote, howev er, that, nowadays, some oth e r ma t e rials are better suit ed f or t his particular t ask [35]. The catalytic properties o f GNPs m ay a ls o b e u s e d f o r C O o x i d a t i o n i n t o C O 2 , using these GNPs i n gas masks and s o me c hemic a l processes, a s we l l a s f o r t h e r e m o v a l o f C O t r a c e s [ 3 5 ] . T h e y m a y a l s o b e u s e d a s d e c o r a t i v e i n k s , o r a s a colouring ad ditive i n nic h e gl ass‐ w a r e a n d c e r a m i c s . T h e s u c c e s s f u l p r i n t i n g o f G N P s p r o d u c e d b y U S P t h u s s h o w s t h e p o s s i b i l i t y of utilisin g t hese GNPs fo r the fa bricat ion o f speci a l ised prod ucts. 4. Conclusion  Nano gold i nk p repared directly f rom a highly c oncentrated G NPs ’ suspension synthesised t h r o u g h U S P s y n t h e s i s , w a s i n v e s t i g a t e d i n t h e p r e s e n t w o r k . R e sults of d iffer e nt c har a cterisa‐ tion showed t hat the high s tability o f the nano g old ink was co nfirmed.  p o t e n t i a l m e a s u r e ‐ m e n t s s h o w e d t h a t G N P s w e r e n e g a t i v e l y c h a r g e d , d u e t o t h e c a p p i n g o f G N P s w i t h P V P s t a b i ‐ lise r, a nd the succe ssf ul functiona lisation of PV P wa s conf irmed with AT R‐FT I R. T he avera g e size of G NPs in n ano gold i nk w as 1 6.7 nms after man u al f iltr ation t hro u gh i njection filters with 0 . 1 µ m p o r e s i z e s . F r o m t h i s c a s e s t u d y , w e c a n c o n c l u d e t h a t t h e n a n o g o l d i n k p r i n t e d p a t t e r n s are hi ghly s table, w hile t he p ri nting specifics can be m odi f i e d f or d iffer e nt a pplicati ons. Th e preparatio n of n ano gold i nk w ith USP and the sel e cted s ynthesi s parameters r equired filtration o f t h e n a n o p a r t i c l e s i n o r d e r t o p r e v e n t c l o g g i n g o f t h e i n k j e t p rinting head. The gi ven initial results also s how a no n‐ continuo us l ayer o f print e d GNPs, indica t i n g t h a t t h e i n k a n d p r i n t i n g p a r a m e t e r s w o u l d h a v e t o b e m o d i f i e d f o r m o r e f a v o u r a b l e r e s u l t s . T h e p r e s e n t e d r e s u l t s d e m o n s t r a t e t h a t t h e p r e p a r a t i o n o f n a n o g o l d i n k s w i t h t h e U S P t e c h n i q u e c a n c o n t r i b u t e i n t h e fabrication of s pecialised p roducts, u seful for different types o f tech nological a pplications s uch as electrochemical and n ano‐ metallic biosensors. C o n s i d e r i n g t h e s u c c e s s f u l s y n t h e s i s o f g o l d n a n o i n k , t h e p r e p aration o f l ower c oncen tra‐ tions o f GNP s with sizes below 10 n m , and a n ev aluation o f th e ir properties , printing characteris‐ t i c s a n d r e a l ‐ l i f e a n a l y s i s f o r their ap plication in p aper‐bas ed s e n s o r s , w i l l b e t e s t e d i n f u t u r e studies. Testing of novel nano gold ink for inkjet printing Acknowledgement This research was funded by the Ministry of Education, Science and Sport, Republic of Slovenia and the European Union, The European Regional Development Fund (ERDF), Early research careers 2.1. We would like to thank Lidija Rozman Zorko for performing the SEM analysis on the GNPs. References [1] Parab, H.J., Chen, H.M., Bagkar, N.C., Liu, R.-S., Hwu, Y.-K., Tsai, D.P. (2010). 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