G. MARSHALL1, 2, O. Voznyy 1, X. DING 1, D. LEPAGE 1, J.J. DUBOWSKI 1, 3

1Département de Génie Électrique et de Génie Informatique, Université de Sherbrooke, Sherbrooke, Québec, 2 Institute for Chemical Process and Environmental Technology, National Research Council of Canada, Ottawa, Ontario 3 Canada Research Chair in Quantum Semiconductors

GaAs-Molecular Interface for Quantum Semiconductor Biosensors

Motivation: Need to develop optical biosensor for rapid and simultaneous detection (< 15 min) of different pathogenic substances at the point of care

RQMP annual meeting, Montreal, May 14, 2007RQMP annual meeting, Montreal, May 14, 2007

SamplesRecipe I Recipe II

GaAs GaAs-T GaAs GaAs-T

I0 13.02 34.49 15.96 43.56

A 33.98 85.06 61.48 120.95

1(hours) 293.55 69.26 226.47 587.37

I0+A 47 119.55 77.44 164.51

Surface Etching and Thiolation Recipes

Recipe I Recipe II

Surface Etching 37% HCl, 1min NH3.H2O/H2O; HCl/ethanol, no ambient exposure

Thiol Deposition 55oC, with 5% NH3.H2O, N2 Room temperature, no NH3.H2O

GaAs passivated with T16 using recipe II shows higher PL signal and

slower PL decay dynamics, therefore more stable interface.

0 400 800 12000

50

100

150

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PL

In

ten

sity

(a.

u)

Atmospheric Exposure Time (hours)

Recipe IIGaAsGaAs-T16 (r.t.)

0 400 800 12000

50

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PL

In

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sity

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Atmospheric ExposureTime (hours)

Recipe IGaAs

GaAs-T16 (55oC)

Nanocrystals and various nanoparticles interfaced with biological materials are thought to have

potential as novel luminescent probes for both diagnostics (e.g., imaging) and therapeutic (e.g., drug

delivery) applications because of their comparable size to biomolecules and attractive optical, electrical

and magnetic properties. Semiconductor quantum dots (QDs) are of particular interest due to their

unique optical properties, including practically unbleachable fluorescence and wide spectrum coverage

(from 400 nm to 2 μm).

Instead of colloid quantum dots, we employ epitaxial grown QDs (eQDs) array grown directly on

different substrates by thin film deposition technology. eQDs arrays allow for implementation of various

processing techniques that otherwise would be impractical for colloidal QDs. Our device will have a far

greater number of simultaneously resolvable QDs emitting distinct spectra specially designed for ultra-

sensitive detection of minuscule quantities of different biomolecules. By binding commercially-available

monoclonal antibodies to separate dots in the multicolour array of QDs, we expect to achieve high

specificity and simultaneous detection of different viruses or viral antigens.

Less oxides were observed in GaAs passivated with T16 using recipe II

AsOxAsOx

48 46 44 42 40 38

Binding Energy (eV)

Recipe IGaAs-S(CH2)15CH3

N2 atmosphere55oC

48 46 44 42 40 38

Binding Energy (eV)

Recipe IIGaAs-S(CH2)15CH3

r.t.

INTRODUCTION

Proposed Architecture of Quantum Dots Based Biosensors

Functionalization of GaAs surface is of

paramount importance for the

application of QDs for biodetectionSubstrate

Epitaxially grown

quantum dots

(eQDs), e.g., InAs

Virus

Antibody GaAs capping layer

SURFACE PASSIVATION OF GaAs WITH CH3(CH2)15SH (T16)

XPS Analysis of T16 Passivated GaAs

Photoluminescence (PL) Decay of T16 Passivated GaAs

1/0

tAeII PL Decay Curves are fitted with

Transmission FTIR Analysis of GaAs-S(CH2)15CH3

Recipe II shows stronger absorption, a narrower peak and lower energy of the vibrational mode corresponding to a higher degree of self-assembly of an ordered monolayer

BIOTIN FUNCTIONALIZATION OF THE GaAs INTERFACE

XPS Analysis of the GaAs-ATA-Biotin Architecture

Chemical shift of the S 2p doublet (left) is -1. 3 eV relative to the free state thiol (163.7 eV) indicating charge transfer to the sulphur and the formation of a covalent bond with the GaAs substrate

Increase of the N1s signal (right) is characteristic of biotin functionalization of the amine headgroups. Coverage of the amine is 43%, though steric repulsion of biotin excludes up to 50% of the available sites

The molecular interface with GaAs provides a physical link to the biosensing elements in addition to stabilization of the surface properties. The proposed architecture is depicted below (left) consisting of a monolayer of ATA (amine-terminated alkanethiol - HS(CH2)11NH2) functionalized with biotin.

FTIR spectra is consistent with molecular ordering of ATA (above right)

CONCLUSIONS

• Effect of deposistion conditions on quality of SAMs is studied theoretically

•Tested recipes show that recipe without ammonia in solution provides better ordered and more stable in time SAMs.

• Same recipe provides similar results for NH2 terminated thiols, avoiding interaction of NH2 group with surface. These SAMS are used for the following biotinilation of the sample.

•XPS results confirm high amount of biotin attached to the surface, appropriate for further attachment of avidin.

FACTORS AFFECTING THIOL DEPOSITION ON GaAs(001) Adsorption mechanism of thiols on GaAs is not well studied and understood. From available experimental data following 3 main factors affecting deposition can be highlighted: Absence of oxides Surface stoicheometry pH of the solution

Our first-principles calculations have shown that chemisorption of thiols happens via cleavage of S-H bond in a physisorbed precursor state. Depending on surface composition this cleavage may be energetically favourable or not. The amount of sites at which it is favourable determines the speed of SAM deposition. Cleaved hydrogen in most cases stays on surface, or can desorb as molecular H2, as on gold.

Thus, the quality of SAM is determined by the success of the preparation technique to remove H from surface (or to avoid its deposition).

Available experimental results fall into 2 categories of SAMs: with 57 tilt and 14 tilt.Our calculations show that 50% thiol – 50% H adsorption results in tilt of 57 , while 100% coverage by thiols results in 14 tilt.