Batch-to-Batch Variation in Laser-Inscribed Graphene (LIG) Electrodes for Electrochemical Sensing

Under the study's four-batch protocol, contact angle varied by less than 5% across batches for both bare LIG and platinum-metallized LIG. The authors conclude: "Contact angle did not vary across batches, but the type of liquid tested did have a significant effect on mean contact angle." The >30% variation reported below occurred only when 36 electrodes were laser-written in a single day without downtime; an intra-day throughput effect, not batch-to-batch variation, and one the authors could not explain: "Additional studies are required to identify the source of the variation during the LIG manufacturing process."

The average contact angle for LIG in DI was reported as 58.6 ± 1.4°, and the authors interpret this as indicating a hydrophilic surface under their test conditions.

The mean contact angle in 2× isotonic buffer was reported as 59.3 ± 2.6°, and described as not significantly different than DI; within fabrication batches, contact angle variation for non-modified LIG in DI and isotonic bicarbonate was reported as less than 5%.

Contact angles for non-modified LIG in HEPES, MES, and Tris were reported as significantly lower than DI, and the authors interpret this as each buffer behaving as a mild surfactant impacting surface tension; within-group variation for these buffer conditions was reported as 6% to 8%.

When 36 electrodes were fabricated in a single day, reported contact-angle variation was more than 30%. Fabricating and analyzing 36 electrodes as four batches of nine with a 30 min laser downtime between batches was associated with reduced variation (reported as a reduction from 30% to less than 5%), and the authors identify laser maintenance, operational frequency, and batch size as control factors for future protocols.

After metallization with nPt, the contact angle in DI and isotonic carbonate buffer was reported as 78 ± 4°, described as an increase by nearly 20% with a more hydrophobic surface. The authors report the HEPES/MES/Tris contact angles for nPt-LIG as similar (63 ± 2°) and higher than all LIG experiments, with within-batch variation described as 5%.

Using oxidative peak current, single LIG electrodes varied by less than 2% across batches; the LIG sensor chip by 8%. Peak separation was 205 ± 50 mV for single electrodes and 519 ± 20 mV for the chip.

Nanoplatinum deposition raised peak current by 150–250 µA, increased capacitance sevenfold, and cut peak separation to 143 ± 3 mV. But batch variation rose to 15–20% within batches and 30–40% across batches. The paper's headline conclusion: "metallization of LIG with nPt improves performance but comes at the expense of increased batch variability."

The authors report that preliminary Young–Laplace (axisymmetric) fitting showed significant errors (up to 60% for the same sample tested multiple times sequentially), and therefore used the polynomial (non-axisymmetric drop) approach throughout for contact-angle calculations.

LIG is a rough, porous carbon film whose "surface chemistry and feature size are highly variable," and polyimide "undergoes deformation during the graphitization process." Axisymmetric fitting assumes a symmetric drop; on this surface the drop pins asymmetrically, which is why the polynomial (non-axisymmetric) method was used for every reported value. The angles reported are apparent contact angles on a rough heterogeneous surface, not intrinsic Young angles. No roughness correction, hysteresis, or advancing/receding measurement was performed.

The study reports markedly different contact-angle variation for 36 electrodes fabricated in a single day versus splitting fabrication into multiple batches with laser downtime, and highlights maintenance and operational factors (e.g., lens cleaning, operational frequency, batch size) as protocol controls.

HEPES, MES, and Tris produced significantly lower contact angles on non-modified LIG than DI, and the authors interpret these shifts in the context of surfactant-like behavior and interfacial effects.

Nanoplatinum metallization increased reported contact angles (78 ± 4° in DI and isotonic carbonate buffer) and shifted contact angles in HEPES/MES/Tris relative to non-modified LIG.

The authors characterize LIG using goniometry together with electrochemical tests (OCP and CV) as part of the overall analysis of batch-to-batch variation for electrochemical sensing applications.