Let us start with silver. Why Ag in place of other raw materials Because it is a noble metal, featuring undisputed advantages in terms of electrical conductivity, resistance to oxidation, and providing interesting plasmonic and antibacterial properties, as we will see further in the text. The topic is far too wide to be synthesized in a single sentence, and there is no single source from which to extract information regarding the different materials that could be used to prepare conductive inks (Au,1 Cu,2 brass,3 nickel,4 Cr,5 Fe,6 Ti,7 intrinsically conductive polymers,8 thin conductive oxides,9 carbon-based materials).10 It is difficult to imagine a future without the use of Ag, at least for certain critical systems that cannot lose efficiency. The market share will be reduced in favor of other nanoengineered, less expensive materials, but it is not possible to avoid the use of metals to transport electricity without losses.
The main route involves the bottom-up synthesis, starting from the silver salts and leading to the final nanocrystals. Three distinct stages can be roughly recognized (Figure 3).32,33 Nucleation, the clustering of few atoms and/or ions, is the first stage of any crystallization process.34 In the second step, a seed is formed through atom-by-atom addition to the initial nuclei. In the final step, the seeds grow mainly in size while the shape is largely determined by the structure of the seed.
Conductive inks normally are aqueous or organic solvent dispersions of silver NPs that are stabilized by surfactants and polymers that undergo printing, a drying step, and at the end a sintering process that is commonly achieved by heating the printed substrates to a temperature usually higher than 200C. Alternatively, more unconventional techniques, such as microwave,64 laser radiation,65 flash sintering,66 plasma,67 and electrical- or chemical-induced sintering, can be pursued.68,69
Sintering at 200C is much below the melting point of silver (960C), and it can be attributed to the enhanced surface diffusion of atoms and to surface premelting; therefore, even in the sintering process, the dimension and shape of Ag NPs are one of the first properties to be investigated.70
Thermal sintering has been discussed by several authors as a method to optimize the quality of printed silver ink lines, in view of their use as electrodes. A critical drying temperature was found to determine an optimal profile of the printed line, thus also improving the electrical properties of the electrode. In these studies, the authors also considered the effect of other factors on the properties of the printed electrodes, such as drop volume,136 different substrates, and thicknesses of the printed layers.137,138
Silver remains one of the best options for application as a conductive ink and adhesive, compared to other electrically conductive fillers. This is mainly due to its high electrical and thermal conductivity, chemical stability, relatively low cost (compared to gold or graphene, for example), and the ability of its oxide form to conduct electricity2. Additionally, silver nanoparticles have a low melting point, which promotes the generation of conductive thin films in relatively low temperatures, this is vital to applications in flexible substrates, such polymers and papers4,7,8. Different methods can be used for the synthesis and stabilization of silver nanoparticles. One of the most popular approaches is chemical reduction, using a variety of organic and inorganic reducing agents7,9. Depending on the method used silver nanoparticles can be fabricated with different morphologies, sizes, shapes, and concentrations10.
Conductive inks are generally formulated with metallic particles (e.g. silver, copper, and gold) or carbon particles (such as graphene and carbon nanotubes) in a retention matrix. The matrix in this application needs to be insulate or a weak conductor. In order to create a contact path with the conductive particles, it is necessary that the volume of the matrix be reduced either with a curing or evaporation process, also called the sintering process2. Organic dispersant and stabilizers are added into the silver nanoparticle conductive ink formulation to prevent agglomeration caused by the high surface energy of these nanoparticles. Sintering decomposes these organic agents, which are used to encapsulate the nanoparticles, allowing these particles to interact. Although there are many sintering techniques, such as thermal, chemical, electric, and laser sintering, these methods result in extra costs and time after the printing process11. As an alternative, substrates surface which have been coated with cationic polymers have been used. These cationic polymers spontaneously sinter the particles. This sudden reaction occurs due to the presence of chloride ions in the coating, which decapsulates the nanoparticle from the deagglomerating agents, promoting the sintering of the particles. Kodak and Epson photo papers both contain this type of coated surface11,12,13.
With that in mind, this study aimed to develop different conductive ink formulations using silver nanoparticles, investigate the formulation properties, fabrication process performance, and the application as electrodes to detect biochemical reactions based on electrical impedance using a buffer solution.
All properties of the material are affected by the ink quality, such as evaporation and film homogeneity. This means that each ink formulation may be suitable for different applications. In general, to formulate conductive inks, the most commonly used materials are particles of silver dispersed in a proper mean, which allow good ink ejection28. Considering that organic materials were applied as solvents, and humectant agents used to increase viscosity are non-conducting, the sample I-1 was formulated only with ethanol to obtain a minimum attainable resistivity. However, this sample was not used in the printing tests because it did not have the required characteristics for printing, as the lack of humectant could clog the nozzles.
Printed electronics will bring to the consumer level great breakthroughs and unique products in the near future, shifting the usual paradigm of electronic devices and circuit boards from hard boxes and rigid sheets into flexible thin layers and bringing disposable electronics, smart tags, and so on. The most promising tool to achieve the target depends upon the availability of nanotechnology-based functional inks. A certain delay in the innovation-transfer process to the market is now being observed. Nevertheless, the most widely diffused product, settled technology, and the highest sales volumes are related to the silver nanoparticle-based ink market, representing the best example of commercial nanotechnology today. This is a compact review on synthesis routes, main properties, and practical applications.
Since 2004, nanoComposix has provided monodisperse and unagglomerated silver nanoparticles to thousands of customers. Hundreds of different variants of size, shape, and surface are available as stock products and we have produced over 2000 custom core/shell, biofunctionalized, fluorescent, and antimicrobial silver nanocomposites to meet client specifications.
Silver nanopaticles are widely incorporated into wound dressings, and are used as an antiseptic and disinfectant in medical applications and in consumer goods. Silver nanoparticles have a high surface area per unit mass and release a continuous level of silver ions into their environment. The silver ions are bioactive and have broad spectrum antimicrobial properties against a wide range of bacteria. By controlling the size, shape, surface and agglomeration state of the nanoparticles, specific silver ion release profiles can be developed for a given application.
Incorporation of silver particles into plastics, composites, and adhesives increases the electrical conductivity of the material. Silver pastes and epoxies are widely utilized in the electronics industries. Silver nanoparticle based inks are used to print flexible electronics and have the advantage that the melting point of the small silver nanoparticles in the ink is reduced by hundreds of degrees compared to bulk silver. When scintered, these silver nanoparticle based inks have excellent conductivity.
Silver nanoparticles have unique optical properties because they support surface plasmons. At specific wavelengths of light the surface plasmons are driven into resonance and strongly absorb or scatter incident light. This effect is so strong that it allows for individual nanoparticles as small as 20 nm in diameter to be imaged using a conventional dark field microscope. This strong coupling of metal nanostructures with light is the basis for the new field of plasmonics. Applications of plasmonic silver nanoparticles include biomedical labels, sensors, and detectors. It is also the basis for analysis techniques such as Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Fluorescent Spectroscopy.
There is increasing interest in utilizing the large scattering and absorption cross sections of plasmonic silver nanoparticles for solar applications. Since the nanoparticles act as efficient optical antennas, very high efficiencies can be obtained when the nanoparticles are incorporated into collectors.
Each batch of silver nanoparticles is extensively characterized using techniques including transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential, and UV-Visible spectroscopy. In addition to ensuring that every batch of nanoparticles meets our stringent quality control requirements, customers are provided with batch-specific specification sheets containing representative TEM images, sizing data, hydrodynamic diameter measurements, zeta potential analysis, UV-Visible spectrum, and solution pH.
Surface chemistry and suspension buffer details are provided for each material,