Pillar Cuvettes: Capillary-Filled, Microliter Quartz Cuvettes with Microscale Path Lengths for Optical Spectroscopy

Abstract

The goal of most analytical techniques is to reduce the lower limit of detection; however, it is sometimes necessary to do the opposite. High sample concentrations or samples with high molar absorptivity (e.g., dyes and metal complexes) often require multiple dilution steps or laborious sample preparation prior to spectroscopic analysis. Here, we demonstrate dilution-free, one-step UV-vis spectroscopic analysis of high concentrations of platinum(IV) hexachloride in a micropillar array, that is, "pillar cuvette". The cuvette is spontaneously filled by wicking of the liquid sample into the micropillar array. The pillar height (thus, the film thickness) defines the optical path length, which was reduced to between 10 and 20 μm in this study (3 orders of magnitude smaller than in a typical cuvette). Only one small droplet (∼2 μL) of sample is required, and the dispensed volume need not be precise or even known to the analyst for accurate spectroscopy measurements. For opaque pillars, we show that absorbance is linearly related to platinum concentration (the Beer-Lambert Law). For fully transparent or semitransparent pillars, the measured absorbance was successfully corrected for the fractional surface coverage of the pillars and the transmittance of the pillars and reference. Thus, both opaque and transparent pillars can be applied to absorbance spectroscopy of high absorptivity, microliter samples. It is also shown here that the pillar array has a useful secondary function as an integrated (in-cuvette) filter for particulates. For pillar cuvette measurements of platinum solutions spiked with 6 μm diameter polystyrene spheres, filtered and unfiltered samples gave identical spectra.

Influence of Sample Volume and Solvent Evaporation on Absorbance Spectroscopy in a Microfluidic “Pillar-Cuvette”

“…We have previously described the fabrication and use of the pillar-cuvette, 5 however, here we review the type of experimental data collected and the previously reported method of analysis. Figure 2a shows a typical spectrum for methyl orange solution with an analytically useful absorbance peak at 464 nm.…”

“…2b and for further details see Ref. 5. This methodology assumes that diffusion between the droplet and the liquid film in the pillar array is negligible (see Ref.…”

“…We have shown elsewhere that the gradual change in concentration caused by evaporation gives a linear increase in absorbance with time, which can be extrapolated back to time zero to ensure a precise measurement. 5 In this study, the rate of evaporation and loaded sample volume were considered in more detail to determine whether this simple relationship is reliable for pillar-cuvette use under varied experimental conditions. The results show that the loaded sample volume (between 1 and 10 μL) and evaporation rates do not adversely affect the absorbance measurements collected.…”

“…A detailed description of sample loading into the pillar cuvette, data analysis method, and other phenomena is described elsewhere. 5 In this study, all measurements were conducted at 30 -40% relative humidity and 23 C.…”

“…We have previously reported a spontaneously-filled absorbance spectroscopy cuvette for spectroscopic detection of samples with very high concentrations or molar absorptivity. 5 The cuvette is open to the environment and, herein, we explore the limitations of such a cuvette with respect to sample loading and evaporation during measurements.…”