Fast and simple assessment of surface contamination in operations involving nanomaterials

Highlights

• Surface contamination caused by handling nanomaterials was traced using fluorescently labeled 80-nm silica nanoparticles.

• Fluorescent silica nanoparticles granted high analytical sensibility, small sizes and high dispersability in aerosol phase.

• Below 100 ppm of fluorescent nanoparticles added to the aerosol matter allowed exposure assessment in wide contact surfaces.

• The appropriate illumination revealed the deposition zones, allowing a low-cost procedure for risk assessment tests.

Abstract

The deposition of airborne nanosized matter onto surfaces could pose a potential risk in occupational and environmental scenarios. The incorporation of fluorescent labels, namely fluorescein isotiocyanate (FITC) or tris-1,3-phenanthroline ruthenium (II) chloride (Ru(phen)₃Cl₂), into spherical 80-nm silica nanoparticles allowed the detection after the illumination with LED light of suitable wavelength (365 or 405 nm respectively). Monodisperse nanoparticle aerosols from fluorescently labeled nanoparticles were produced under safe conditions using powder generators and the deposition was tested into different surfaces and filtering media. The contamination of gloves and work surfaces that was demonstrated by sampling and SEM analysis becomes immediately clear under laser or LED illumination. Furthermore, nanoparticle aerosols of about 10⁵ nanoparticles/cm³ were alternatively fed through a glass pipe and personal protective masks to identify the presence of trapped nanoparticles under 405 nm or 365 nm LED light. This testing procedure allowed a fast and reliable estimation of the contamination of surfaces with nanosized matter, with a limit of detection based on the fluorescence emission of the accumulated solid nanoparticles of 40 ng of Ru(phen)₃@SiO₂ of silica per mg of non-fluorescent matter.

Introduction

Thanks to the unique properties of nanomaterials, the global nanotechnology market is expected to grow at a compound annual growth rate of around 17% during the period 2017–2024 [1], giving rise to a variety of nanotechnology-enabled products. These same new properties may give rise to new risks and uncertainties and the scientific community has been aware of the possibility of nanomaterials having an adverse impact on human health or the environment for more than two decades [[2], [3], [4], [5], [6], [7], [8]]. The main concern, however, has always been the inhalation of nanoparticles [[9], [10], [11]] and indeed measuring nanoparticle aerosol concentration is part of the standard procedure in any assessment of exposure to nanoparticles in the workplace and the environment [[10], [11], [12], [13]]. Fortunately, a number of commercial instruments have become available to measure number and size distribution in nanoparticle aerosols, solving one of the challenges identified by Maynard et al. [14] in the development of sustainable nanotechnologies. Also, current engineering controls and personal protective equipment have been shown to be highly effective at providing worker protection during the handling of nanomaterials [15,16].

However, there are other aspects that are often overlooked when considering possible exposure scenarios. Among these, the inadvertent contamination of surfaces that could have been exposed to nanoparticles stands out as the main concern, on account of two main reasons. The first aspect relates to the contact of unprotected skin with surfaces that are contaminated with nanoparticles. It is generally accepted that one should not use presumably contaminated gloves when touching surfaces in common areas: the one-glove policy (see for instance [17]) and similar approaches imply that bare hands should be used to touch common-area surfaces such as handles, computer and instrument keyboards, electrical switches and so on. If these surfaces were contaminated with nanoparticles, anyone touching them unprotected would be subjected to skin contact with nanoparticles with subsequent risks associated to different entry routes: dermal, ingestion, etc. It is thus necessary to have some means of detection of nanoparticle contamination on common-area surfaces that can be touched unprotected. However, detection of nanomaterials contamination on surfaces is usually a laborious procedure. In previous works [18,19], we used rare earth labeling of TiO₂ nanoparticles to monitor surface contamination during handling operations followed by sampling and TEM-EDX analysis or elemental analysis (ICP-OES) of digested wipes after cleaning the suspect area. Similarly, Hedmer et al. [20] used adhesive tape sampling followed by electron microscopy to detect contamination by carbon nanotubes and nanodiscs of work surfaces at a small-scale manufacturer. All of these works involve complex procedures requiring expensive instruments and specialized personnel.

In addition to skin contact, the second aspect of concern is that nanoparticle-contaminated surfaces themselves may become points of generation of secondary aerosols that could be subsequently inhaled. The opportunities are ample and include handling of polluted laboratory items (lab coats, masks, instruments), operation on surfaces close to handling areas outside fume hoods (laboratory benches close to weighting stations are a common example), and cleaning of laboratory surfaces and instruments, which is often carried out by external (usually non-scientific) personnel. Studies related to the generation of aerosols from work surfaces and clothing contaminated by nanomaterial handling are extremely rare. Ding et al. [21] recently reviewed worker exposure during different handling activities. They pointed out that some low-energy activities, including cleaning, could result in the release of airborne nanoparticles. Tsai [22] investigated the release of nanomaterials associated to the handling of clothing contaminated with nanoparticles and found that the release of airborne nanoparticles depended on the type of fabric, with cotton showing the highest release levels. Schneider et al. [23] discussed the dustiness and degree of agglomeration of nanomaterials in relation to personal exposure, and also recognized the role of contaminated surfaces as potential sources of secondary aerosols. All of these studies provide valuable insight on the phenomena involved in respirable aerosol generation from contaminated sources and on the complexity of the evaluation of the risks involved, but do not provide a simple method to spot this contamination.

Here we present a simple procedure to detect surface contamination caused by operations involving the handling of nanomaterials. We have adapted a synthesis procedure to obtain un-aggregated, fluorescently labeled silica nanoparticles with a particle size around 80 nm. These nanoparticles present several important advantages as markers of surface contamination: (i) the fluorescent molecules are trapped in the core of the particle, i.e. the surface consists essentially of unmodified silica; (ii) the particles are highly fluorescent and this allows detection even at very low concentration in the presence of a strong dilution by non-fluorescent background material; (iii) their small size and un-aggregated nature means that they will be dispersed easily in handling operations and travel longer distances. Fluorescent labeling is a well-known technique to visualize and quantify the nanosized matter in cell studies and biological media [24]. Because of these characteristics, fluorescent-labeled unaggregated particles constitute excellent candidates as added-on tags to other nanomaterials in risk assessment of operations involving the handling of nanomaterials. Thus, to evaluate the extent of contamination caused by a specific handling operation (e.g. weighting, cleaning laboratory synthesis equipment, bagging of nanosized materials) a small proportion of fluorescent nanoparticles would be added to the material being handled. Subsequently, a simple inspection under laser or LED light illumination would reveal the area likely affected by the contamination. The results obtained are surprisingly sensitive and provide an easy, low-cost procedure to identify likely contaminated areas during risk assessment exercises.