Air pollution is considered to be one of the most serious environmental problems worldwide because of its impact on human health [1, 2]. Particulate matter (PM), a major contributor to air pollution, is a heterogeneous mixture of solid and liquid elements in the air [3, 4]. Inhalation of PM into the body can cause severe respiratory diseases, such as asthma, pneumonia, and even lung cancer, and can also increase the severity of existing diseases [5, 6]. Social interest in the effects of PM is expanding to various research fields, such as identifying and predicting occurrence phenomena, developing detection devices and emission reduction technologies, and evaluating the health effects of PM to preserve human life.
The effect of PM on the human body is compounded by the degree of its toxicity according to the constituents and effects depending on its size or morphology [7–9]. The constituents of PM are distributed in various ways depending on the cause of occurrence and secondary pollution according to the air flow, and it is difficult to generalize the effects of PM toxicity on the human body due to variations among countries and regions in various environments [10]. However, these toxicity issues are strongly influenced by the size of PM [11]. A generalization of the impact of PM size on human health could significantly facilitate the prediction of its toxicity.
PM exhibits different deposition and distribution patterns in different respiratory tracts depending on its size [12]. Understanding the biological distribution of PM can enable prediction of its effects on human organs, as the toxic effects on surrounding organs depend on the pathway through which it travels and the duration of residence and accumulation after inhalation.
Based on the general criteria of aerodynamic diameters, PM is classified into coarse particles with a diameter of less than 10 µm (PM10), fine particles with a diameter of less than 2.5 µm (PM2.5), and ultra-fine particles with a diameter of less than 0.1 µm (PM0.1) [7, 9]. Larger (PM10) particles are primarily deposited by inertial impaction in large airway regions, including the oropharynx, trachea, and bronchi. The deposition of smaller particles in the trachea and bronchi occurs primarily through fast inhalation [13, 14]. PM2.5 particles are deposited via gravitational sedimentation along smaller airways, and particles smaller than 0.5 µm in diameter reach the alveoli by Brownian diffusion and are deposited there [15, 16].
Recent studies have demonstrated that ultra-fine particles have greater mass-based toxic effects than large particles. However, there is an urgent need for research on the effects of small particles
[17]. Although information on the biodistribution and assessment of the health effects of fine particles is available in several reports, only in vivo or ex vivo imaging using radioisotope-labeled or optical fluorescent-labeled materials as PMs has been attempted in a fragmented manner [14, 18]. Biodistribution analysis of fine or ultra-fine particles requires a comprehensive analysis including in vivo tracking along with in vitro tissue analysis to investigate particle interactions at the cellular level.
In this study, silica-based particles, which are highly amenable to engineering and modification, were used as model materials to simulate particulate matter, and near-infrared (NIR) dyes suitable for bioimaging were doped inside them to study their distribution in vivo and ex vivo. In particular, PM0.1 particles, small particles of less than 100 nm in diameter, and particles 2 µm or larger in diameter were separately synthesized and employed for the comparative analysis of distribution patterns. In addition, quantitative analysis of tissue images was performed and cellular uptake information was observed over an extended period (Fig. 1a).