*3.2. Metal Nanoparticles (NPs)*

#### 3.2.1. Material Properties, Synthesis, and Ink Formation

AgNPs and CuNPs are low aspect ratio particles, typically with spherical geometries and radii from 10–100 nm for printing applications, that are often formed through wet chemistry from ionic precursors [85]. Example images of printed Ag and Au nanoparticles with spherical geometries are shown in Figure 6a [94]. In wet chemistry NP synthesis, a metal ion precursor, such as AgNO<sup>3</sup> and Cu(NO3)2, is reacted with a reducing agent, such as ethylene glycol (EG) or ascorbic acid in solution with a capping agent, such as PVP and SDS [85]. In addition to wet chemical synthesis, NPs may also be formed through physical methods, such as evaporation condensation [95] and laser ablation [96], additional chemical methods, such as microemulsion [97], UV or other photonic source initiated photoreduction [98], electrochemical synthesis [84], irradiation [99], microwaveassisted synthesis [100], and biosynthesis techniques, either through bacteria, fungi, algae or plants [100]. Spherical metal NPs tend to agglomerate strongly because of their large surface areas, strong interparticle attractions, and particle symmetry regardless of orientation. As a result, the NP surface must be functionally modified to aid in dispersion. Furthermore, their low aspect ratios require high material loadings in order to form conductive networks, but loadings over 60% complicate the design of inks for printing methods requiring low viscosities or which tend to clog, such as inkjet printing. On the other hand, the excellent material symmetry, high material loading, and surface pre-melting allow for very effective, low-temperature sintering at around 200 ◦C into uniform conductive films. An example of

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a printed AgNP film is provided in Figure 6b, and the resultant AgNP network after the solvent is dissolved is readily seen [101].

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**Figure 6.** Metal nanoparticles for printed electronics. (**a**) photos and SEM images of inkjet-printed films with Ag (left) and Au (right) NPs. (reprinted under Creative Commons license CC BY 4.0 from *Adv. Radio Sci.* (2019), 17:119–127.) (**b**) SEM images of an AgNP deposition cross-section, showing the overall structure (**i**), surface (**ii** and **iv**) and interior (**iii** and **v**) after rapid laser sintering. (reprinted with permission from *Appl. Sci.* (2020), 10(1), 246. Copyright 2019, MDPI). (**c**) SEM images of AgNP films sintered at various temperatures, with corresponding graphs depicting the coefficient of variance and resistivity (reprinted with permission from *Materials* (2011), 4(6), 963–979, Copyright 2011, MDPI).

NPs inks are typically synthesized with 40–88% material loadings and dispersed with high concentrations of dispersants, such as 1:1 PVP mixtures. Because this drastically reduces the ink viscosity, PVP concentrations must be limited for screen printing. For instance, Wang et al. preheated and magnetically stirred a 0.3 M solution of PVP and ethylene glycol (EG) to increase the ability of PVP to bind to the AgNP surface, allowing them to disperse the NPs with a 1:2 PVP/AgNO<sup>3</sup> ratio [102]. After mixing 60 mL of 0.3 M PVP-EG solution and 40 mL 0.29 M AgNO3-EG, the solution was mixed with N,N-dimethylformamide, hydroxyethyl cellulose, and ethylene glycol (EG) to yield a 45 wt.% ink with viscosity and rheology optimized for screen printing. When printed on PI, and sintered at 220 ◦C, the inks demonstrated a remarkably low resistivity of 8.3 <sup>×</sup> <sup>10</sup>−<sup>6</sup> <sup>Ω</sup>·cm, which is only five times greater than the bulk silver resistivity [102]. For gravure printing, Shiokawa et al. created an organic protection layer on AgNPs to improve dispersibility and printability. AgNO<sup>3</sup> (22 wt.%) was mixed with oxalic acid dihydrate (9 wt.%), n-Hextlamine, N,N-dimethyl-1,3diaminopropane and oleic acid, and AgNPs were synthesized through thermal decomposition of an oxalate-bridged silver alkylamine complex [103]. The resultant powder was then dispersed in tetralin, tetradecane, and dodecane with 80 wt.%, and it was determined that the tetralin solution had the highest printability [103]. The ink was then gravure printed on a glass slide with widths of 20 µm with 4.4 µΩ cm [103]. For flexographic printing, Benson et al. developed an AuNP ink that was used to create biocompatible sites on a PI substrate for the enzyme attachment in glucose sensing [104]. AuNPs were synthesized by reducing HAuCL<sup>4</sup> (0.2 g) with NaBH<sup>4</sup>

(0.05 g) in the presence of PVP (0.15 g) in 30 mL DI. The solution was centrifuged to yield an AuNP pellet, which was subsequently redispersed in 70% IPA and 30% dionized water (DI) via ultrasonication. Electrodes were fabricated by flexographic printing of a carbon layer, then the AuNP layer, with a printing force of 125 N, anilox force of 125 N, and speed of 0.6 m/s. After functionalization with glucose oxidase, the electrodes demonstrated a high sensitivity of 5.52 µA mM−<sup>1</sup> cm−<sup>2</sup> with a detection limit of 26 µM [104]. In addition, NPs are highly attractive for inkjet printing because of their low aspect ratios, which can avoid nozzle clogging, and they have thus been carefully studied [26]. For instance, Fernandes et al. designed an experiment to assess the printability and conductivity of AgNP inks with a variety of solvents and additives [47]. Silver nanoparticles were synthesized by reduction of 100 mL 0.006 M AgNO<sup>3</sup> and 0.008 M PVP in DI water by 8 mL of 0.529 M sodium borohydride (NaBH4), centrifuged at 1500 RPM for 1 h, then dispersed in a range of ethanal based solutions with viscosities ranging from 3.7–7.4 mPa.s and material loadings from 8–16 wt.% [47]. It was determined that the EG, ethanol, ethanolamine, and hyperdispersant (Solsperse 20000) ink with 5.25 mPa.s viscosity resulted in the greatest printability due to the addition of humectants (i.e., ethylene glycol and ethanolamine) combined with low resistivity (1.6 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>Ω</sup>.cm) [47] Finally, AgNPs are not well suited to skin contact because of poor biocompatibility, so they either must be well insulated or replaced with AuNPs for such applications [26].
