1. Introduction

Paper-based electronics represents an emerging field that leverages paper as a flexible, inexpensive, and biodegradable substrate for devices such as batteries, transistors, and sensors. Silver serves as the primary material for creating conductive paths on these substrates, owing to its high stability and superior electrical conductivity compared to carbon-based alternatives.

There is a critical need for universally adaptable and low-cost techniques to translate laboratory innovations into accessible solutions, particularly for the developing world. Conventional methods often rely on expensive substrates or energy-intensive processes, limiting their practicality.

Traditional printed electronics face several hurdles: conductive inks require complex formulations, and nanoparticle-based inks often necessitate high-temperature sintering or chemical curing to achieve conductivity. Moreover, many existing methods demand smooth, specialized substrates, which escalate manufacturing costs and hinder widespread adoption.

The primary objective of this work is to develop a method for creating precisely patterned, highly conductive silver layers on standard, untreated copier paper. A core challenge lies in ensuring a clean metallization process that avoids residual solid by-products, which could impede conductivity.

This project employs a "print-expose-develop" cycle inspired by the historic "salt printing" photographic process. A modified desktop inkjet printer sequentially deposits silver precursors, followed by UV light activation and chemical development, resulting in a conductive silver nanowire network formed directly on the paper fibers.

In contrast to the original report, which primarily focused on demonstrating the underlying photographic chemistry, the present work explicitly emphasizes translating this lab-scale chemical process into a low-cost, accessible printing technique that can be implemented with standard cartridge-based inkjet hardware.
Furthermore, we introduce a process–device compatibility perspective by systematically considering factors such as printhead limitations, ink viscosity, and layer uniformity, thereby linking the chemistry of silver halide formation and development directly to the practical requirements of paper-based electronic devices rather than treating it as a purely materials-chemistry problem.

2. Literature Review

Silver-based functional inks are central to printed and flexible electronics because silver provides the highest electrical conductivity among stable metals and exhibits good chemical robustness under ambient conditions. Existing silver inks used for printing conductors can be broadly categorized into three classes: organometallic inks, complexed silver salt-based inks, and nanoparticle-based inks.

Organometallic silver inks incorporate silver in an organometallic precursor that decomposes upon heating to form metallic silver on the substrate. Although such inks can yield conductive traces, they typically require relatively high decomposition temperatures, which are incompatible with thermally sensitive substrates such as ordinary cellulose paper. Consequently, their usefulness for low-cost paper-based electronics is limited by substrate degradation, discoloration, and increased energy consumption.

Complexed silver salt-based inks contain silver salts (for example, silver nitrate) in a complexed form, together with suitable reducing agents, to promote in situ formation of overlapping silver nanostructures during drying or mild heating. These systems can achieve conductivities comparable to that of bulk silver and are attractive for both printed and hand-drawn conductors. However, they suffer from short shelf-life, sensitivity to environmental conditions, and tedious multi-step processing, which collectively restrict their applicability in low-resource or field settings.

Silver nanoparticle-based inks, consisting of pre-formed silver nanoparticles dispersed in a solvent and stabilized by organic capping agents, represent the most widely commercialized class of conductive inks. After printing, curing steps such as thermal annealing, microwave treatment, plasma sintering, or chemical sintering are required to remove organic stabilizers and fuse particles into continuous conductive paths. While these inks have been optimized for smooth polymeric substrates, they pose several challenges for paper-based electronics, including the need for smooth or pre-treated substrates, complex ink formulation, and potential nozzle clogging, especially in conventional ink-tank printers.

Paper has attracted considerable attention as a substrate for low-cost, disposable electronic devices including batteries, field-effect transistors, RFID antennas, sensors, and solar cells. In many of these systems, paper serves both as a mechanical support and as a porous matrix that can host electrolytes or conductive networks, enabling mechanically flexible and biodegradable device platforms. To exploit these advantages, there is a strong push towards additive manufacturing technologies—particularly digital inkjet printing—that can pattern functional materials on paper with minimal waste and without lithographic masks.

Despite the maturity of inkjet technology, the integration of conventional silver inks with paper remains nontrivial. Organometallic inks demand high curing temperatures, complexed salt-based inks require careful handling and have limited stability, and nanoparticle-based inks depend on smooth substrates and often rely on costly post-deposition sintering infrastructure. In addition, typical “ink-tank” printer architectures require large ink volumes (7–13 mL per tank), which is wasteful when printing experimental nanomaterial formulations and increases the risk of permanent printhead damage due to nanoparticle clogging.

An alternative paradigm is provided by the classical silver halide photographic process, particularly salt printing, in which silver halide layers are formed in situ on paper, exposed to light to create a latent image, and subsequently chemically developed to yield a metallic silver pattern. Parmar and Santhanam exploited this concept to demonstrate in situ formation of conductive silver nanowire networks on ordinary office paper using a “print–expose–develop” cycle. In their approach, aqueous potassium halide and silver nitrate solutions are sequentially deposited onto paper by an office inkjet printer to form densely packed layers of silver bromide; after exposure to light and immersion in a photographic developer, the silver halide crystals disintegrate into a percolating network of silver nanowires that conformally coat the paper fibres. The resulting films exhibit sheet resistances of a few ohms per square and specific conductivities on the order of 1–10% of bulk silver, with good mechanical robustness under repeated bending.

The present work builds on this photographic-metallization strategy but differs from the original report in two important aspects. First, a cartridge-based inkjet system (Canon PG-745 cartridges) is used instead of a fixed ink-tank configuration, which significantly reduces the required precursor volume and isolates the reactive salt solutions from the printer’s internal ink delivery system. This choice minimizes chemical waste and lowers the risk of irreversible printhead damage, thereby making the process more practical for laboratory settings where frequent formulation changes are required. Second, ultraviolet (UV) exposure at 254 nm is employed in place of broadband halogen lamp exposure. UV photons at this wavelength couple efficiently to the absorption bands of silver halides and enable rapid formation of latent silver clusters at relatively low radiative power. By combining cartridge-based inkjet deposition with UV-induced photographic development, the proposed methodology retains the advantages of the salt-printing approach—no nanoparticle synthesis, no complex sintering steps, and compatibility with standard office paper—while improving operational simplicity and accessibility for low-cost printed electronics.

In addition, our study explicitly incorporates optimization of precursor molarity within the constraints of inkjet printability, whereas the reference work reports an effective parameter set without systematically discussing how viscosity and flow behavior of the salt solutions limit the usable concentration window.

We also examine the effect of printing order of the precursor solutions (AgNO₃-first versus KX-first) on the final conductivity, a variable that is not deeply explored in the original paper but turns out to significantly influence in situ precipitation and microstructural uniformity in our experiments.

3. Materials and Methodology

3.1 Chemical precursors and compositions

Silver nitrate (AgNO₃) and potassium halide (KX, where X = Br⁻, I⁻) are employed as precursor salts to generate silver halide layers in situ on paper. AgNO₃ provides soluble silver ions, while KX supplies halide ions that precipitate silver halide according to the reaction Ag++X- → AgX(S). The molar ratio of KX to AgNO₃ is maintained at 2 : 1 during printing, ensuring near-complete conversion of silver ions to silver halide and minimizing the presence of residual soluble silver species in the paper matrix. This stoichiometric excess of halide is important for controlling the initial crystal phase and for avoiding uncontrolled nucleation during the development step.

The halide composition is chosen as 95 wt% bromide and 5 wt% iodide. A small fraction of iodide is known from photographic science to significantly enhance the photosensitivity of silver halide emulsions by generating deeper electron traps and highly active sensitivity centers. In the present context, the bromide–iodide mixture yields silver bromide–iodide (AgBr:AgI) microcrystals that are more responsive to UV irradiation than pure AgBr, enabling efficient latent image formation at moderate exposure doses.

In the reference process, typical solution concentrations range from 0.167 to 1.67 M for AgNO₃, with corresponding KX concentrations from 0.33 to 3.3 M, and three sequential printing passes produce a silver loading of approximately 1.04 mg/cm². In this work, patterns are printed at 600 dpi with three consecutive layers to approach and exceed the percolation threshold necessary for macroscopic conductivity while preserving pattern fidelity.

In our implementation, we further explored a narrower, practically relevant molarity window for AgNO₃ in the range of approximately 1.5 M to 2.5 M, while maintaining the 2 : 1 KX : AgNO₃ molar ratio.
Within this range, we identified an optimal AgNO₃ concentration of about 2.25 M, at which the printed features exhibited a favorable balance between high silver loading (and hence low sheet resistance) and reliable jetting without satellite droplets or nozzle misfires.
We observed that increasing molarity generally led to better conductivity due to higher silver halide loading, but beyond the optimal point the accompanying increase in solution viscosity and changes in surface tension began to degrade print quality and pattern uniformity, effectively imposing a process-limit on further gains in conductivity.
While the Parmar and Santhanam study reports an optimized condition for conductivity, it does not explicitly address these inkjet-specific viscosity–flow constraints, which are critical for translating the chemistry into a robust printing protocol on commercial cartridge-based systems.

3.2 Developer solution and component functions

A D-76-type photographic developer is employed as the chemical reducing bath for transforming the latent silver halide image into a metallic silver nanowire network. As reported by Parmar and Santhanam, substitution of D-76 with ascorbic-acid-based developers yields films with significantly lower conductivity, typically about 50% of that obtained using D-76, indicating that the redox environment and kinetics provided by the D-76 formulation are more favorable for forming dense, interconnected nanowire networks.

The D-76 developer solution is prepared according to a standard recipe, with the following composition per litre:

Sodium sulphite: 100 g

Metol (monomethyl-p-aminophenol hemisulphate): 2 g

Hydroquinone: 5 g

Borax: 2 g

Distilled water: to make 1,000 mL

The resulting pH of the developer is approximately 8.5.

Each component of the D-76 formulation plays a distinct role:

Metol is a primary, moderately active reducing agent that initiates development by reducing Ag⁺ ions in regions containing latent silver clusters. Its relatively gentle activity favors fine-grain development, leading to numerous nucleation points rather than large, isolated silver grains. This behavior is conducive to the formation of multiple wire-like protrusions from the parent silver halide crystals.

Hydroquinone is a more energetic developer that acts synergistically with Metol. After initial reduction by Metol, hydroquinone supplies electrons at a faster rate, amplifying the latent image and driving continued reduction of mobile silver ions throughout the crystal volume. This sustained reduction promotes disintegration of the original AgBr:AgI grains into filamentous silver structures that extend outward and bridge between neighboring crystals, thereby establishing long-range electrical connectivity.

Sodium sulphite functions primarily as a preservative. It scavenges dissolved oxygen and inhibits oxidative degradation of Metol and hydroquinone, thereby maintaining the activity of the developer over the entire development period. Additionally, sodium sulphite influences the solvation environment of the silver halide surface and can subtly affect grain morphology, indirectly contributing to the uniformity and fineness of the resultant nanowire network.

Borax (sodium tetraborate) serves as a buffering agent to maintain the developer in a mildly alkaline regime, with pH near 8.5. The reduction kinetics and redox potentials of Metol and hydroquinone are strongly pH-dependent; too low a pH suppresses their activity, while excessively high pH increases the risk of fogging and uncontrolled reduction. Borax provides a stable pH environment, ensuring reproducible reaction rates and minimizing unwanted side reactions while dipping the printed paper in the developer solution.

Distilled (deionized) water is used as the solvent to avoid contamination by extraneous ions, carbonates, or particulates that could introduce uncontrolled nucleation sites, alter pH, or deactivate the developer components. The use of DI water is important for achieving reproducible conductivity and nanostructure formation across different batches.

Other classes of photographic developers, such as phenidone-based formulations or ascorbic acid (ascorbate) developers, were considered. Phenidone developers provide very high activity and rapid development but require tight control to avoid fogging and coarse grain formation. Ascorbic-acid-based developers are more environmentally benign but, in this specific system, produce lower conductivity due to less favorable control over nanowire morphology and network density. The D-76 formulation offers a balanced combination of fine-grain development, buffer stability, and proven compatibility with silver bromide systems, justifying its selection for this work.

3.3 Cartridge modification and inkjet printing protocol

Standard Canon PG-745 inkjet cartridges are modified to enable printing of aqueous precursor solutions. The cartridge lid is carefully opened, and the internal sponge that normally retains commercial ink is removed. The reservoir is thoroughly rinsed with tap water followed by multiple rinses with distilled water until test prints show no residual black coloration, indicating that dyes, surfactants, and other additives from the original ink have been effectively removed. This procedure converts the cartridge into a clean reservoir that can be filled with either KX or AgNO₃ solutions without introducing contaminants that might interfere with silver halide formation or development.

Patterns are created using vector graphics software and printed at a resolution of 600 dpi. Under typical office printer conditions, this resolution corresponds to a droplet spacing of approximately 42 µm and an average droplet volume on the order of a few picolitres. Each pattern is printed in three successive passes to increase the areal loading of silver and to improve spatial uniformity. In each pass, the KX solution and the AgNO₃ solution are printed sequentially in the same patterned region, preserving the overall 2 : 1 molar ratio of KX : AgNO₃ and promoting immediate in situ precipitation of AgBr:AgI microcrystals within the paper fibres.

In addition to using the KX-then-AgNO₃ sequence described above, we systematically examined the effect of reversing the printing order by first depositing AgNO₃ followed by KX in otherwise identical three-pass prints.
Experimentally, the configuration in which AgNO₃ was printed first and the KBr/KI solution was overprinted yielded consistently lower sheet resistance and more uniform conductive tracks than the reverse order, suggesting that the local supersaturation dynamics and penetration depth of the ions within the paper matrix are sensitive to the sequence in which the precursors encounter each other.
This printing-order dependence is not deeply explored in the reference paper but is crucial from a process-engineering standpoint, because it links cartridge assignment and printhead firing sequence directly to the microstructure and connectivity of the resulting nanowire network.

3.4 Ultraviolet exposure

Following printing, the paper bearing AgBr:AgI layers is exposed to ultraviolet radiation from a low-pressure mercury lamp operating at 254 nm with a power of 50 W. Exposure times in the range of 15-20 minutes are employed. The UV photons are absorbed by the silver halide crystals, generating electron–hole pairs; the electrons become trapped at lattice defects or iodide-rich sensitivity centers, where they reduce nearby Ag⁺ ions to form nanometer-scale metallic silver clusters (latent image specks). The presence of iodide enhances this process by creating deeper traps and more active sensitivity sites, thereby increasing the efficiency of latent image formation.

The amount of UV exposure is critical. As observed in the reference work using halogen lamps, insufficient exposure leads to incomplete and non-uniform nanowire formation, resulting in large variations in local resistance, while exposure beyond a certain dosage does not significantly further improve conductivity. The chosen exposure window of 15-20 minutes at 254 nm balances the need for a sufficient density of latent silver clusters with the desire to avoid overexposure and unnecessary energy input.

3.5 Development, fixing, and washing

After UV exposure, the printed paper is dipped in the D-76 developer bath. During this stage, the latent silver clusters act as catalytic centers for reduction. Metol and hydroquinone donate electrons to these sites, which then reduce mobile Ag⁺ ions within the AgBr: AgI lattice to metallic silver. As reduction proceeds, silver atoms accumulate preferentially at the latent image sites, and anisotropic growth leads to filamentous nanowire outgrowths from the original polygonal crystals. The nanowires bridge the gaps between adjacent crystals, and once they establish contact, they can induce further metallization in neighboring grains, ultimately yielding a three-dimensional, percolating silver nanowire network embedded within the paper fibres.

Upon completion of development, the samples are transferred to a 0.5 M sodium thiosulphate (hypo) solution for 10 minutes. Thiosulphate complexes and dissolves remaining unreacted silver halide, thereby removing residual light-sensitive and ionic silver species from the film. This fixing step improves the chemical stability of the conductive patterns without significantly altering their electrical properties. After fixing, the paper is thoroughly rinsed with distilled water to remove residual thiosulphate and developer components and is then dried either under ambient conditions or using gentle warm air. The resulting films exhibit good adhesion of the silver nanowire network to the paper substrate and maintain stable resistance values under repeated bending, consistent with previously reported performance.

4. Working Principle

The working principle of the process is rooted in classical silver halide photographic chemistry, adapted to produce conductive silver nanowire networks rather than merely optical images. Initially, the sequential deposition of AgNO₃ and KX produces a compact layer of AgBr:AgI crystals anchored within the porous paper matrix. Exposure to UV light generates electron–hole pairs in the silver halide; electrons are trapped at defects and iodide-rich sites, where they reduce nearby Ag⁺ ions to form metallic silver clusters that constitute the latent image.

During development in the D-76 solution, these latent silver clusters serve as electron reservoirs and catalytic nuclei. Metol and hydroquinone supply electrons that further reduce mobile Ag⁺ ions located within the AgBr:AgI lattice. Rather than forming simple, isotropic grains, the reduction process is strongly anisotropic, leading to filamentous outgrowths of metallic silver nanowires with diameters in the range of 30-100 nm. As these nanowires elongate from the surfaces of the parent crystals, they can span the inter-crystal gaps and contact neighboring grains, which in turn become metallized. Through this cascade, the original non-overlapping silver halide microcrystals are converted into an entangled, percolating nanowire network that extends through the thickness of the paper.

The evolution of electrical conductivity can be described in terms of percolation theory. As the silver nitrate loading is increased, a critical threshold of approximately 130±20 μg/cm2 is reached, at which the sheet resistance of the film drops sharply by about five orders of magnitude, from ~10⁸ Ω/sq. to ~10³ Ω/sq. Above this percolation threshold, the sheet resistance follows a power-law dependence on the silver loading, with an exponent consistent with three-dimensional percolating networks. The resulting submicron-thick films achieve specific conductivities on the order of 1-10% of bulk silver and exhibit only modest changes in resistance (a factor of about three) after 1,000 bending cycles, demonstrating that the nanowire network is both electrically and mechanically robust.

In summary, the process combines inkjet-mediated in situ silver halide formation, UV-induced latent image generation, and D-76-based chemical development to produce conductive silver nanowire networks on ordinary paper. The use of a cartridge-based printer and a D-76 developer allows this methodology to be implemented with simple, widely available equipment while delivering conductive patterns suitable for low-cost, flexible paper electronics.

5. Results and Discussion

5.1 Observations

Visual changes (Color and deposition)

Upon UV/sunlight exposure, printed regions exhibited a distinct color transition from pale/whitish to brown and eventually dark brown/black.
Samples from lower molarities (1.5–2.0 M) predominantly showed light brown coloration, indicating partial reduction of silver halides and incomplete network formation.
The 4th trial (~2.25 M) produced uniform dark brown to black regions, suggesting efficient conversion of silver halides to metallic silver and the formation of a dense, percolating silver nanowire network.
In some cases, patchy or uneven coloration was observed, indicating non-uniform overlap of precursor layers during printing.
The 5th trial (2.5 M) showed inconsistent and faint deposition, despite higher concentration, due to poor ink flow.

Printing quality

At lower molarities, printing was smooth but resulted in lower deposition and weak-contrast patterns.
At moderate molarities (~2.25 M), the system showed the best balance between printability and deposition, and the patterns were sharp, continuous, and well-defined.
At higher molarity (2.5 M), solution viscosity increased significantly and ink flow through the cartridge nozzles was restricted.
This resulted in broken patterns, non-uniform deposition, and poor reproducibility.
Sequential printing order also influenced quality: printing halide solution first tended to give better in situ precipitation within the paper matrix, whereas printing AgNO₃ first led to more spreading and less controlled reaction.

5.2 Analysis

Effect of processing variations

a) Developer vs no developer

Samples treated with the D-76 developer showed much darker coloration and significantly lower resistance, indicating extensive reduction of silver halides and the formation of a denser silver network.
Samples processed without the developer remained lighter and showed poorer conductivity, consistent with only partial photoreduction of Ag⁺ to metallic silver under UV/sunlight alone.
This confirms the critical role of the developer in promoting nanowire growth and establishing a well-connected three-dimensional network.

b) UV vs sunlight exposure

UV exposure at 254 nm produced a faster and more uniform color change, enabling better control over the extent of photoreduction and network formation.
Sunlight exposure was slower and less consistent, leading to more variability in the final patterns and, in some cases, underdeveloped regions.

c) Printing order

When AgNO₃ was printed first and the halide solution was deposited afterwards, silver halides formed immediately at the sites where the two solutions overlapped, leading to better localization and higher microstructural uniformity.
When KI/KBr was printed first, the halide solution tended to spread more deeply into the paper before encountering Ag⁺, resulting in less controlled precipitation and more diffuse reaction zones.

Why the results behaved this way

The system follows classical silver halide photochemistry: silver nitrate reacts with halide ions to form silver halides; UV exposure generates a latent image by forming small clusters of metallic silver; the developer then amplifies these latent centers and drives growth into an extended nanowire network.
A key aspect is the change in oxidation state of silver: Ag⁺ (Ag(I)) in the silver halide lattice is reduced to Ag⁰ (metallic silver) at latent image sites, and this Ag⁰ serves as a catalytic center for further reduction and anisotropic growth.
Higher precursor molarity increases the amount of silver halide available for reduction, which generally improves conductivity by enabling more continuous pathways, but it also increases viscosity and alters surface tension, which can negatively affect droplet formation, jetting stability, and wetting.

Viscosity vs flow trade-off

At 2.5 M, the solution became too viscous for the cartridge to handle reliably, restricting flow through the internal pores and nozzle orifices.
As a result, the effective deposited mass of silver halide decreased locally despite the higher nominal concentration, leading to incomplete coverage and discontinuous tracks.

Percolation threshold

Around 2.25 M, the combination of adequate silver content and acceptable printability enabled the formation of continuous conductive pathways across the printed area.
This corresponds to the percolation threshold regime, where small increases in silver loading cause a sharp drop in resistance as isolated metallic domains become interconnected.

Developer chemistry

The D-76 developer promotes anisotropic growth of metallic silver by preferentially extending filaments from latent nuclei rather than producing isotropic, compact grains.
This anisotropic growth is essential for nanowire formation and leads to better interconnectivity between neighboring grains, thereby enhancing macroscopic conductivity and mechanical robustness.

5.3 Limitations

Despite the successful demonstration of in situ silver nanowire network formation, several limitations were encountered during the course of this project.

Scale of characterization

The process involves the formation of nanostructures on the order of 10⁻⁹ m, while the evaluation in this work was primarily conducted at a macroscopic level using visual observation and bulk electrical behavior.
This creates a gap between the actual structure (nanowire geometry and morphology) and the observed performance (color, sheet resistance, and qualitative conductivity).
Without advanced characterization, it is difficult to confirm nanowire dimensions and distribution or to verify crystallinity and phase transformation.
Ideally, techniques such as scanning electron microscopy and X-ray diffraction should be employed to bridge this gap and enable microscale (10⁻⁶ m) to nanoscale analysis, providing a clearer structure–property correlation.

Infrastructure and workspace constraints

The project required a wet-lab setup with continuous monitoring, which was not always readily available.
Limited access to a dedicated workspace affected experimental consistency and the storage and handling of chemicals and samples.
Since this was among the first wet-lab synthesis projects under IEEE, infrastructure and procedural readiness were still evolving.

Time-intensive and presence-dependent process

The methodology involves multiple sequential steps: printing, UV exposure, development, fixing, and washing.
Each step demands careful timing and physical presence, making the process labor-intensive and difficult to scale within limited timeframes.
Iterative trials involving multiple molarities and printing conditions further increased the overall time requirement.

Logistical and administrative constraints

Practical challenges extended beyond the lab, such as storage of the modified printer in hostel rooms, which required administrative permissions.
Institutional procedures occasionally caused delays and interruptions.
These constraints reduced effective experimental time and slowed down iteration and optimization cycles.

Printing system limitations

Standard inkjet cartridges are not designed for high-molarity chemical solutions.
Issues observed included increased viscosity at higher molarity, nozzle clogging and inconsistent flow, and misalignment during sequential printing.
These factors limited uniform deposition and reduced the reproducibility of results.

While these limitations restricted deeper structural analysis and process optimization, they also highlight the practical challenges of translating laboratory-scale chemistry into accessible, low-cost fabrication techniques.

6. Validation / Proof of Concept

Validation test using NaCl

To confirm that the observed brown/black patterns were due to silver-based reactions and not artifacts from ordinary ink, a qualitative validation test was performed using sodium chloride solution.

Principle

Free silver ions from silver nitrate react with chloride ions to form silver chloride, a characteristic pale white precipitate; this reaction serves as a confirmatory test for the presence of silver species.

Observations

Upon applying NaCl solution to the printed samples, regions containing unreacted silver salts showed localized pale/whitish discoloration, consistent with AgCl formation.
The discoloration was confined to specific patches, indicating non-uniform presence of residual Ag⁺, likely due to printing overlap variations.
In contrast, when NaCl was applied to ordinary brown ink, no precipitate formation was observed.
Instead, chromatographic separation occurred: the ink spread and separated into different tones because multiple dye components have different solubilities and mobilities in water.

The localized nature of AgCl formation also provides insight into regions with incomplete reaction or excess precursor, supporting earlier observations on printing non-uniformity.

7. Applications

Printed electronics: low-cost interconnects and conductive traces on disposable substrates.
Flexible circuits: paper-based or hybrid paper–polymer platforms for simple circuitry and prototyping.
Low-cost sensors: resistive or electrochemical sensing elements integrated onto paper for environmental or point-of-care applications.
This newly developed inexpensive method using a highly accessible substrate will be very helpful for future paper-electronics-related devices and research, especially where the combination of high conductivity, chemical stability, and compatibility with photographic development strongly favors silver over carbon-based alternatives, which typically exhibit lower conductivity and may require different processing chemistries.

8. Future Scope

Future work will focus on enhancing conductivity through process optimization, improving substrate compatibility for better pattern fidelity, and employing advanced characterization techniques like XRD and SEM to establish a clear structure–property relationship in the in situ formed silver nanowire networks.

Morphological analysis

Scanning electron microscopy (SEM) can be used to visualize nanowire formation in the 30–100 nm range, confirm the presence of a percolating network structure, and help correlate morphology with conductivity.

Electrical characterization

Four-point probe measurements can provide accurate sheet resistance values without contact resistance errors.
Mapping conductivity across printed patterns will help assess spatial uniformity and reveal process-sensitive regions.

9. Conclusion

This project successfully demonstrates a low-cost, scalable approach for fabricating conductive silver nanowire networks on ordinary paper using a modified inkjet printing technique.
By employing a print–expose–develop strategy inspired by photographic chemistry, sequential deposition of silver nitrate and potassium halides enabled the in situ formation of photosensitive silver halide layers, which upon UV exposure and chemical development were converted into a percolating metallic silver network.
The incorporation of a bromide–iodide mixture enhanced photosensitivity, enabling efficient latent image formation under UV irradiation.
Subsequent development using a D-76 formulation facilitated the growth of interconnected silver nanowires, resulting in conductive pathways embedded within the paper matrix.
An important outcome of this work was the optimization of precursor molarity for inkjet compatibility.
It was observed that while higher molarity solutions improve silver loading and thereby enhance conductivity, they also significantly increase solution viscosity.
Beyond a certain limit, this increased viscosity restricts smooth flow through the cartridge nozzles, leading to poor or inconsistent printing.
Thus, an optimal molarity range was identified that balances printability and conductivity, which is critical for practical implementation using standard inkjet systems.
The study highlights that conductivity is strongly governed by percolation behavior, where sufficient silver loading and proper overlap of printed layers are critical for achieving continuous conductive networks.

Acknowledgement

We would like to express our sincere gratitude to Dr. Keyur (HoD, Chemical Engineering Department) for generously providing access to his laboratory as our working space and for his constant support throughout the course of this project.
We also extend our thanks to the Quality Testing Lab, Chemical Engineering Department, for supplying the necessary chemicals and equipment, without which this work would not have been possible.
We are especially grateful to Rupesh Sir from the Indian Institute of Science for his valuable guidance, insightful suggestions, and continuous mentorship, which greatly contributed to the successful completion of this project.

References

Parmar, In situ silver formation paper.

Tadaaki Tani, Photographic Science: Advances in Nanoparticles, J-Aggregates, Dye Sensitization, and Organic Devices.

Shinsaku Fujita, Organic Chemistry of Photography.

https://iopscience.iop.org/article/10.1088/2058-8585/aab725/meta

Canon Deskjet TS207 properties – https://share.google/K3WAtw21G07iL2zCU

D-76 developer properties – https://share.google/O68O2AqY8ICcoGG5W

Keywords: Paper-based electronics; silver nanowire networks; in situ metallization; inkjet printing; cartridge-based inkjet cartridges; silver nitrate–potassium halide precursors; silver halide photochemistry; D-76 photographic developer; UV-induced reduction (254 nm); molarity–viscosity optimization (1.5–2.5 M, optimal ~2.25 M); printing order effects (AgNO₃-first vs KX-first); percolation-driven conductivity; flexible conductive patterns; low-cost fabrication; validation with NaCl test; limitations and future nanoscale characterization.