From the poster titled:Monodispersed nanochannels for sub-nanometric filtration leveraging high

spatial confinement

Giacomo Bruno, R. Lyle Hood, Giancarlo Canavese, and Alessandro Grattoni

Department of Nanomedicine, Houston Methodist Research Institue, Houston, TX 77030

Abstract

Nanochannel fluidic systems offer new opportunities for molecular filtration, protein sorting, bio-analytical separation, and desalinization/water purification. Common filtration methods leverage stochastic distributions that diminish filtration quality while also depending on the chemical compatibility of the solution with the filter. In this study, we demonstrated the use of industrially fabricated, silicon-based nanochannels enabling a new digital filtration technique with an accuracy of 0.5 nm. This work provides an initial demonstration of the sub-nanometric spatial resolution in sorting and filtering permitted by silicon nanochannels within chemically reactive environments.

Methods

• Quantification of fabrication accuracy:• Membranes were fabricated following standardized industrial

techniques of silicon as described by [1, 2]• TEM images were taken to quantify the uncertainty caused

during the manufacturing process• Quantification of filtration resolution:• CdSe core quantum dots (QDs), with 2.0, 3.0, and 4.0 nm

diameter, dissolved in toluene were employed • Rated emission peaks at 507, 535, and 584 nm for the

respectively QDs were acquired using a fluorospectrophotometer

• The experiment was conducted with 2.5 and 3.6 nm nDS membranes and repeated five times (n=5)

• The measurements were normalized to aid comparison with the 2 nm DQ release, and analyzed with the t/test to detect the statistically relevant differences

Quantification of fabrication accuracy

The architectural structure of the nDS membrane and nanochannels was analyzed with the SEM and TEM techniques. The images confirming the size of channel was corresponding to the nominal values from 2.5 nm to 200 nm in height. The precision of the fabrication method is exhibited in Fig C-I and evident in the smallest channels (with an error less than 10% of the nominal value). This ultra-high fidelity employment of a well developed and reliable manufacturing technique allowed significant encroachment into previously unreachable nanoscale resolutions with sufficient density to permit clinically-relevant therapeutic outflow.

Quantification of fabrication accuracy

The high resolution TEM images lacked the field-of-view to quickly measure a sufficient population size among the high density nanochannels as it required individual focus and a destructive cross-section cut for every layer into the chip. The quantum dot experiment was specifically designed to overcome these limitations.

Quantification of filtration resolution

• QDs represented a superior alternative in measurement accuracy, as their emission peak was a function of their size, and detectability, as their high emission intensity eased quantification of low concentrations. The filtration was conducted in a system composed by two stainless steel chambers and Viton® O-rings resistant to the toluene required for QDs suspension.

• The figure represents the spectrum distribution of QDs after three days of filtration with a 3.5 nm membrane. The fluorescents peaks corresponding at the 2 and 3 nm QDs are clearly visible, while the concentration of the 4nm QD is not detectable.

Quantification of filtration resolution

The figure represents the normalized release of different QDs. The findings are:• 2.5 nm nanochannels: a significant drop in release between the

2.0 and the 3.0 nm QDs is evident, while there was no significant difference between the 3.0 and the 4.0 nm QD populations

• 3.6 nm nanochannels: a statistically relevant contrast is present between the 3.0 and 4.0 QDs, while the 2.0 and the 3.0 populations did not differ significantly

The * represent a statistically relevant differences between two different QD dimension

Summary

Various filtration approaches have been demonstrated to facilitate precise control of molecules/proteins sorting, including continuous flow sieving [3], and tunable filtration actively controlled by electric field or mechanical forces [4].This group has demonstrated that meticulous silicon-based manufacture allows the fabrication of a nanochannel systems to achieve sub-nanometer filtration while maintaining relevant rates. These extraordinary membranes were produced with monodispersed slit-nanochannels as tight as 2.5 nm at a density of greater than 100,000/mm2 through refined sacrificial etching methods. Nanochannel sizes were confirmed through TEM cross-sections and selective filtering of neutral-charge quantum dots. Furthermore, we established a novel approach for the use of QDs as a discriminator of sorting capability as their emission peak directly correlates with their dimensions. Chemically resistant silicon nanochannels were unaffected by toluene, a harsh organic solvent used to solubilize the non-functionalized QDs, presenting new filtration possibilities.

Conclusions

• The nDS devices were proven to be:• Highly defined geometries with a fabrication

error within 10% of the nominal value • The silicon-based manufacture proved to be

chemically inert to harsh organic solvents such as toluene

• The filtration spatial resolution was found to be sub-nanometric (0.5 nm or less)

• The filtration resolution, added to the fabrication fidelity and the chemical stability, can lead to filtration and/or sorting capable of accurate molecule selection independent of the surrounding media.

References

• 1. Fine, D., et al., A robust nanofluidic membrane with tunable zero-order release for implantable dose specific drug delivery. Lab Chip, 2010. 10(22).

• 2. Grattoni, A., et al., Nanochannel technology for constant delivery of chemotherapeutics: beyond metronomic administration. Pharm Res, 2011. 28(2).

• 3. Regtmeier, J., et al., Continuous‐flow separation of nanoparticles by electrostatic sieving at a micro‐nanofluidic interface. Journal of separation science, 2011. 34(10).

• 4. Huh, D., et al., Tuneable elastomeric nanochannels for nanofluidic manipulation. Nat Mater, 2007. 6(6).

Acknowledgements

The authors are grateful to Lee Hudson, Ryan Medema, and Randy Goodall (NanoMedical Systems, Inc.) for their support in the chip fabrication and characterization. The authors thank Yuri Mackeyev for his knowledge in molecule charges and properties.