Surfactant-Free Conductive Inks

• Surfactant-free conductive inks are advanced formulations engineered via optimal solvent selection and exfoliation methods to maintain colloidal stability without contaminant residues.
• They employ techniques such as liquid-phase and electrochemical exfoliation to produce films with enhanced electrical properties and low contact resistance.
• Applications span thin-film transistors, flexible sensors, and edible electronics, offering pristine interfaces and simplified processing for high-performance devices.

Surfactant-free conductive inks are formulations designed to enable high-performance printing or direct-deposition of electrically conductive materials where the dispersion stability, functional performance, and electronic quality are achieved without the presence of surfactants or polymeric stabilizers. The rationale for surfactant-free processing is to avoid contaminant residues that degrade conductivity, dielectric properties, contact resistance, and overall device reliability. Recent advances encompass a range of materials (graphene, carbon nanotubes, liquid metals, layered chalcogenides, hybrid 2D frameworks, and edible composites) using solvent-optimized exfoliation, interfacial engineering, physical phase manipulation, and green chemistry techniques.

1. Fundamentals of Surfactant-Free Ink Formulation

Surfactant-free conductive inks rely on either optimal solvent selection, physical exfoliation, transient or removable polymeric processing aids, or intrinsic interfacial chemistry to maintain colloidal stability and printability. For example, in the ink-jet printing of graphene electronics, graphite is exfoliated in N-methylpyrrolidone (NMP), a solvent whose surface energy closely matches that of graphene, satisfying the minimal enthalpy of mixing ($\Delta H_{\mathrm{mix}}$) required for thermodynamic stability ($$\Delta G_{\mathrm{mix}} = \Delta H_{\mathrm{mix}} - T\Delta S_{\mathrm{mix}}$$) without surfactant addition (Torrisi et al., 2011). Similarly, solvent exchange techniques exploiting differential boiling points, such as the DMF-to-terpineol route, enable the concentration of exfoliated graphene to levels suitable for film fabrication with no surfactant contamination (Li et al., 2012). In aqueous systems, stabilization and rheological tuning may be achieved with biopolymer integration (e.g., carboxymethylcellulose sodium salt) rather than conventional surfactant emulsifiers (Karagiannidis et al., 2016).

The absence of surfactants is often necessary for pristine electronics; surfactants can act as insulating barriers, degrade flake-to-flake contacts, and leave residues detrimental to charge transport and interfacial phenomena.

2. Exfoliation and Stabilization Methods

Conductive inks from 2D and layered materials are commonly produced via liquid-phase or electrochemical exfoliation, high-shear microfluidization, ultrasonic or tip-sonication, or controlled galvanic reactions:

• Liquid Phase Exfoliation (LPE): Graphite is sonicated in NMP or DMF, producing single- and few-layer graphene where the solvent's surface energy minimizes $\Delta H_{\mathrm{mix}}$, enabling surfactant-free colloidal stability (Torrisi et al., 2011, Li et al., 2012).
• Electrochemical Exfoliation: Graphene flakes are generated via an applied voltage across graphite foil in aqueous electrolytes (e.g., $(\mathrm{NH}_4)_2\mathrm{SO}_4$), collected, dispersed, and processed entirely in water. Subsequent brief tip-sonication yields sub-micrometer flakes (Parvez et al., 2019).
• Solvent-Assisted and Ultrasonic Exfoliation of MoS$_2$: Bulk MoS$_2$ is sonicated in 1-cyclohexenyl pyrrolidine (CHP) for extended periods to yield two-dimensional sheets, which are freely paintable as ink without additives (Carroll et al., 2020).
• Microfluidization: Turbulent flow at high shear rates ($\sim$10$^8$ s$^{-1}$) achieves quantitative exfoliation of graphite. Polymer stabilizers such as carboxymethylcellulose are added post-exfoliation, avoiding interface-blocking surfactants (Karagiannidis et al., 2016).
• Galvanic Replacement Reaction: Liquid metal (gallium-based droplets) is coated with a conductive silver shell by ultrasonic-assisted redox reaction, replacing native oxide shells and directly yielding printable conductive composites (Hajalilou et al., 29 Jan 2025).

Stable dispersions are often achieved by engineering the ink's viscosity, polarity, and particle size, enabling subsequent printing (inkjet, blade-coating, extrusion, aerosol-jet) without sedimentation or phase separation—and, crucially, without persistent surfactant residues.

3. Electrical and Optical Performance Characteristics

Surfactant-free conductive inks exhibit performance metrics dictated by their composition, exfoliation method, and post-processing:

Ink System	Sheet Resistance (Ω/□)	Conductivity (S/m)	Optical Transmittance (%)
Graphene/NMP (Torrisi et al., 2011)	~30,000	—	~80
Graphene/Terpineol (Li et al., 2012)	~6,000	—	~60
ECG Water Ink (Parvez et al., 2019)	—	~3.9×10$^4$	—
Graphene Aerogel (Gaur et al., 2020)	—	—	—
MoS$_2$ (Carroll et al., 2020)	7.7–26.3×10$^3$	1.2–1.6×10$^{-5}$	—
CNT Polymer-Free (Owens et al., 2021)	—	up to 10,000	—
Liquid Metal LM (Zheng et al., 2013)	—	~3.36×10$^6$	—
Oleogel (Edible) (Cataldi et al., 2022)	—	—	—
LM–Ag/SIS Composite (Hajalilou et al., 29 Jan 2025)	—	—	—

Electrical transport is often dominated by flake percolation phenomena ($\sigma \propto (t-t_c)^{\epsilon}$), with intrinsic properties preserved due to surfactant exclusion. Thin-film transistors printed from surfactant-free graphene inks demonstrate mobilities up to 95 cm$^2$V$^{-1}$s$^{-1}$ (Torrisi et al., 2011); blade-coated films from microfluidized graphite achieve Rs < 2 Ω/□ (Karagiannidis et al., 2016); direct-write CNT traces resistivity under repeated mechanical bending with less than 5% resistance change (Owens et al., 2021); liquid metal GaIn$_{24.5}$ tracks exhibit metallic conductivity ($\rho_{e} \sim 2.98 \times 10^{-7}$ Ω·m) and are free of insulating stabilizer-induced losses (Zheng et al., 2013).

Optical transmittance (critical for transparent electrodes) can reach ~80% in graphene films with Rs ~30 kΩ/□ (Torrisi et al., 2011) and ~47% in porous liquid metal structures with Rs ~5 Ω/□ (Zhang et al., 2013).

4. Device Fabrication and Applications

Surfactant-free inks have enabled a range of high-quality printed electronics:

• Thin-Film Transistors (TFTs): Surfactant-free graphene inks fabricated via LPE/NMP yield TFTs with high charge carrier mobility (up to 95 cm$^2$V$^{-1}$s$^{-1}$) and organic-hybrid TFTs with ON/OFF ratios $\sim 10^5$ (Torrisi et al., 2011).
• Micro-Supercapacitors: Pristine graphene aerogel inks produce micro-supercapacitors (μ-SC) on flexible polyimide with areal capacitance of 55 μF/cm$^2$, negligible cyclic voltammetry distortion at high scan rates, and ∼80% capacity retention over 10,000 cycles (Gaur et al., 2020).
• Printed Sensors, Circuits, and Contacts: Direct-write printed contacts from surfactant-free silver inks on layered materials (graphene, MoS$_2$, Bi-2212, Fe$_5$GeTe$_2$) produce Ohmic interfaces with contact resistance competitive with resist-based lithography, supporting pristine gate voltage response, superconducting transitions, and magnetic phenomena (Jois et al., 6 Mar 2025).
• Flexible/Stretchable Electronics: LM–Ag/SIS composites provide EMI shielding effectiveness exceeding 75 dB (X-band) under 200% strain, signal-blocking for Bluetooth communications, and thermal interface applications with $\kappa \sim 65.2$ W/m·K (Hajalilou et al., 29 Jan 2025).
• Edible Electronics: Oleogel pastes of beeswax, oil, and activated carbon enable scalable, solvent- and surfactant-free fabrication of edible electrodes for impedance-based fruit ripening monitoring and food safety applications (Cataldi et al., 2022).
• Printable MoS$_2$ Films: Aspect-ratio-dependent conductivity in painted MoS$_2$ films facilitates direct electrode patterning on arbitrary surfaces without polymeric additives (Carroll et al., 2020).

5. Interfacial, Rheological, and Environmental Considerations

The exclusion of surfactants yields benefits and obligations in processing:

• Interfacial Cleanliness: Surfactant-free inks yield lower contact resistance, improved charge injection, and cleaner interfaces—especially vital for field-effect devices and layered quantum materials (Jois et al., 6 Mar 2025).
• Printability and Rheology: Proper ink rheology arises from solvent viscosity, polymeric dispersion aids (removable at post-process), and intrinsic particle interactions. Print fidelity (resolution, adhesion, pattern uniformity) is ensured via match of solvent surface energy (e.g., NMP) and post-deposition annealing to remove residuals.
• Environmental and Biological Safety: Green fabrication (e.g., edible pastes, eco-friendly galvanic reactions, deep eutectic solvent recycling) ensures suitability for biomedical, food-monitoring, and large-scale manufacturing regimes (Cataldi et al., 2022, Hajalilou et al., 29 Jan 2025).

Potential drawbacks include control over particle aggregation, reproducible flake size distribution, and, for painted or direct-write films, anisotropy in conductivity due to orientation and aspect-ratio effects (Carroll et al., 2020).

6. Comparative Assessment and Domain-Specific Impact

When compared with surfactant-laden or polymeric inks, surfactant-free systems exhibit:

• Higher Electronic Quality: Absence of insulating and mobile molecular stabilizers avoids degradation of charge transport and enhances intrinsic mobility (Torrisi et al., 2011, Parvez et al., 2019).
• Superior Contact Reliability: Direct-print contacts avoid polymer residues, ensuring consistent Ohmic behavior and low contact resistance for precise device characterization (Jois et al., 6 Mar 2025).
• Enhanced Applicability: Surfactant-free formulations permit implementation in applications where cleanliness (quantum transport, quantum sensing), optical transparency, biological compatibility, or stretchability are required (Owens et al., 2021, Cataldi et al., 2022, Hajalilou et al., 29 Jan 2025).
• Simplified Processing: Elimination of post-deposition surfactant removal steps reduces process time, cost, and environmental risk.

Nevertheless, a plausible implication is that certain systems (aqueous CNT inks (Owens et al., 2021)) may still require transient surfactant use for dispersion, highlighting that the critical metric is removal and absence of persistent residue in the final device state.

7. Future Research Trajectories and Open Challenges

Research is actively addressing:

• Scalability: Microfluidization and solvent-exchange routes are being industrialized for bulk ink production (yielding >9 tons/year) (Karagiannidis et al., 2016).
• Integration with 2D Hybrids: The growth of metal-carbon frameworks (CoGLC) with ordered atomic features, enabling surfactant-free electrochemical exfoliation and ink formation, foreshadows advances in spintronics and catalysis (Ryzhkova et al., 19 Oct 2025).
• Environmental Recycling: Biodegradable solvent-based recovery and electronic recycling align with regulatory, economic, and sustainability mandates (Hajalilou et al., 29 Jan 2025).
• Domain Expansion: Edible and biocompatible ink systems expand applications into point-of-care diagnostics, food monitoring, and transient electronic textiles (Cataldi et al., 2022).

Challenges remain in optimizing the trade-offs between ink concentration, printability, device integration, and achieving high conductivity—especially in 2D chalcogenide systems, edible composites, and emerging hybrid materials.

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Surfactant-free conductive inks thus represent a confluence of interface science, advanced synthesis, and pragmatic engineering, supporting flexible, transparent, quantum, wearable, and edible electronics across a broad spectrum of substrates and device architectures. Their development continues to impact next-generation electronic, sensing, and energy storage technologies.