1. Introduction

Olfactory sensing is gaining attention as computation moves closer to the body and environment. Unlike light or temperature, scent arises from complex mixtures of volatile organic compounds (VOCs), each a distinct molecule, that together create a perceived odor signature rather than mapping to a single chemical component.

Replicating this sense requires converting chemical interactions into digital patterns, a field known as electronic olfaction or digital nose technology.

This document summarizes the current state of the art, focusing on the sensor technologies most relevant for compact, low-power, and design-integrated products. It concludes with the rationale for selecting carbon nanotube (CNT)–based sensors as the foundation for Oloris, a scent-recognition device under development at Gentle Systems.

2. Principles of Digital Olfaction

Digital noses detect and classify VOCs through three stages:

1. Sensing layer — an array of sensors coated with materials that react differently to various gases.
2. Signal conditioning — conversion of analog responses into digital signals.
3. Pattern recognition — ML models map signal patterns to known odor classes.

Key challenges: cross-sensitivity, sensor drift, humidity interference, and reproducibility.

Modern systems mitigate these through software calibration and multi-sensor data fusion.

3. Metal Oxide Semiconductor (MOS) Sensors

Principle

MOS sensors are the most widely used gas-sensing technology. They detect volatile compounds by measuring changes in electrical resistance that occur when the target gases interact with a heated metal-oxide surface. Common base materials include tin dioxide (SnO₂), zinc oxide (ZnO), titanium dioxide (TiO₂), and tungsten oxide (WO₃). These are often doped with small amounts of catalytic metals such as palladium (Pd) or platinum (Pt), typically combined with molybdenum disulfide (MoS₂), a two-dimensional transition metal dichalcogenide semiconductor. Catalysts like pure MoS₂, Au–MoS₂, Pd–MoS₂, or Pt–MoS₂ improve reactivity and selectivity. The MOS sensing layer adsorbs gas molecules, altering its electrical resistance. When saturated, it enters a regeneration phase, typically heated to around 400 °C to desorb the gas and restore baseline conductivity. Recent methods such as Temperature-Cycled Operation (TCO) vary the temperature dynamically, allowing different VOCs to adsorb and desorb at distinct intervals, improving selectivity without adding new materials.