Visualisation of natural aquatic colloids and particles ? a comparison of conventional high vacuum and environmental scanning electron microscopy

Abstract

The applicability of environmental scanning electron microscopy (ESEM; imaging of hydrated samples) and conventional high vacuum scanning electron microscopy (SEM; imaging of dried samples at high vacuum) for the observation of natural aquatic colloids and particles was explored and compared. Specific attention was given to the advantages and limitations of these two techniques when used to assess the sizes and morphologies of complex and heterogeneous environmental systems. The observation of specimens using SEM involved drying and coating, whereas ESEM permitted their examination in hydrated form without prior sample preparation or conductive coating. The two techniques provided significantly different micrographs of the same sample. SEM provided sharper images, lower resolution limits (10 nm or lower), but more densely packed particles, suggesting aggregation, and different morphological features than ESEM, suggesting artefacts due to drying. ESEM produced less easily visualised materials, more complex interpretation, slightly higher resolution limits (30-50 nm), but these limitations were more than compensated for by the fact that ESEM samples retained, at least to some extent, their morphological integrity. The results in this paper show that SEM and ESEM should be regarded as complementary techniques for the study of aquatic colloids and particles and that ESEM should be more widely applied to aquatic environmental systems than hitherto.

A Technological Understanding of Biofilm Detection Techniques: A Review

“…It can thus be concluded that SEM by itself is not capable of generating 3D images of biofilms. Although SEM is not capable of visualizing the thickness and the 3D structure of the biofilm, it can provide a spatial resolution of up to 10 nm [ 81 ]. However, Doucet et al (2005) did not focus on the application of SEM for biofilms [ 81 ].…”

“…Although SEM is not capable of visualizing the thickness and the 3D structure of the biofilm, it can provide a spatial resolution of up to 10 nm [ 81 ]. However, Doucet et al (2005) did not focus on the application of SEM for biofilms [ 81 ]. Moreover, Chatterjee et al (2014) and Merino et al (2019) stated that the maximum resolution that can be obtained by the application of SEM to image biofilms was 50 nm [ 77 , 78 ].…”

“…Moreover, unlike SEM, sample preparation to apply TEM is a time-consuming process and a tedious procedure that requires trained laboratory workers, which is due to the necessity of a very thin sample, a vacuum environment, and the absence of artifacts such as precipitates or amorphization [ 76 , 89 ]. Generally, TEM can be used on hydrated biofilms; however, in practice, multiple articles state that drying is necessary to obtain a sample thickness with a maximum of 150 nm [ 76 , 81 , 91 ]. Since a biofilm in a bioreactor is hydrated, this technique is deemed to be ex situ for bioreactor applications.…”

“…The high water pressure used in ESEM enables imaging of the hydrated specimen, unlike SEM, which can only image dry samples [ 14 , 89 ]. Furthermore, it does not depend on the high vacuum requirements like SEM [ 81 ]. The direct study of fully hydrated or electrically non-conductive dry samples in their native state, without the necessity of a conductive coating, is possible due to high gas pressure, mostly water vapor, in the ESEM specimen chamber [ 92 ].…”

Instrumental approach toward understanding nano-pollutants

“…They also found that the maximum relative moisture for imaging was 75%, since at higher moisture levels, layers of free water on the sample affect colloid visualization. Finally, they concluded that SEM and ESEM ought to be used as complementary techniques [32].…”