A. Kerr

The early stages of marine biofouling and its effect on two types of optical sensors

This paper records the results of an investigation into the effects of biofouling on optical marine sensors and the organisms responsible for the deterioration in sensor accuracy. Two kinds of commercial sub-surface optical marine sensor, commonly used to measure water quality, were operated in a natural marine environment and allowed to foul while measurements of the actual conditions were made daily using clean instruments. A number of glass and acrylic coupons were placed in the same environment and were removed at intervals throughout the trial. These coupons were examined and the biological population quantified. Deterioration in the optical properties of the coupons was measured using image analysis and UV-visible spectroscopy. The results from the coupons were compared with the results from the commercial instruments. It was observed that the major deterioration in instrumental accuracy occurred when a bacterial population exceeding 105/mm2 was found on the coupons. The algal population had little effect on the instruments over this time period. The acrylic coupons supported a lower fouling population, apparently due to the increased solubility of acrylic in seawater. The two optical techniques returned similar patterns of results for the surface area fouled, although the numerical values returned by each technique were different. Neither of these two techniques returned values directly comparable with the deterioration in accuracy of the commercial instruments. The trial took place on the Isle of Cumbrae in the Firth of Clyde on the west coast of Scotland, U.K.

The effect of benzalkonium chloride concentration on nine species of marine diatom

The effect of varying concentrations of benzalkonium chloride (BAK) on nine diatom species is measured, and related to the variation in tolerance levels of different species seen elsewhere in the literature. Different species showed different effective levels; however, all species were non-viable at 1×10−3% BAK. The technique being laboratory based and, therefore, immune from seasonal influences, is quick to perform and is easily adapted for bioassay work.

A novel technique to prevent bacterial fouling, using imposed surface potential

A. KERR, T. HODGKIESS, M.J. COWLING, C.M. BEVERIDGE, M.J. SMITH AND A.C.S. PARR. 1998. The effect of modest imposed surface potentials on the adhesion of marine bacteria to an electrically conducting layer deposited on silica glass is recorded. A positive shift increased bacterial settlement. However, a negative shift in potential was extremely beneficial in reducing numbers of adhered bacteria. An applied surface potential of − 66 mV SCE resulted in the bacterial population decreasing to approximately 12% of that on the uncharged reference sample. There was no further significant decrease in the adhered bacterial population when the magnitude of the negative potential was increased. The potential was maintained with very little current flow (less than 0·25 nA mm−2). The results were not due to any effect of the material used and therefore the technique could be useful for reducing bacterial fouling in many situations, including medical applications.

Optimising optical port size on underwater marine instruments to maximise biofouling resistance

The limiting effects of biofouling have restrained the widespread deployment of optical marine sensors. This has necessitated the development of biofilm resistant coatings that are not detrimental to the quality of any measurement recorded. A comprehensive study has been carried out into the effect of changes in the diameter of optical ports. It was found that increasing the diameter of an optical port in the range 10-46 mm reduced the biofouling per unit area. Changes in the diameter did not affect the release of an antimicrobial agent from a hydrogel-based coating. The results show that the operational lifetime of optical ports is significantly improved for diameters over 30 mm and it is suggested that this should be considered a preferred design minimum for optical ports, regardless of the size of the underlying sensor.

The biofouling resistant properties of six transparent polymers with and without pre-treatment by two antimicrobial solutions

Six bulk polymers potentially suitable for use as optical ports of underwater instruments were exposed to a solution of marine bacteria after soaking in distilled water or surfactant solutions. The effect of the surfactant solutions was to reduce fouling build-up on four of the six polymers. The presence of the surfactant altered the surface energy of the polymers. The surfactant reduced the importance of physical characteristics, such as surface roughness, on fouling build-up. It was found that untreated polyethylene terephthalate out-performed polymethyl methacrylate, over short time periods. This result was repeated when these polymers were tested on optical underwater instruments exposed to a marine environment.

Some physical factors affecting the accumulation of biofouling

The effects of surface roughness and microsolubility on fouling levels are examined using glass and acrylic samples. It is found that both of these, often overlooked, physical characteristics have a noticeable effect on the rate of fouling. The microsolubility of acrylic results in lower fouling than found on glass despite the higher hydrophobicity of acrylic and the resultant increase in initial attraction for fouling organisms. Fouling levels were found to increase with increasing surface roughness and therefore studies on the fouling susceptibility of different materials should report the roughness values of the samples examined.

Optical assessment of a fouling‐resistant surface (PHEMA/ benzalkonium chloride) after exposure to a marine environment

A hydrogel based on poly(2-hydroxyethyl methacrylate) (PHEMA) containing benzalkonium chloride (BAK) can be used as an environmentally acceptable, fouling-resistant material in the marine environment. The loaded hydrogel system is transparent and has the potential to be used in the protection of optical ports in underwater instruments. Ultraviolet–visible (UV–vis) spectroscopy was used to study the optical properties of the material after a marine exposure period. The optical transmittance of PHEMA/ BAK was higher for 10 weeks than that detected for poly(methyl methacrylate) (PMMA), a material currently used in commercial instruments, which confirmed the superior fouling resistance of the PHEMA/ BAK combination. The UV–vis spectroscopic method was quick, relatively cheap and accurate enough to allow the effects of the development of marine fouling on transparent surfaces for use in marine underwater optical applications to be monitored.

Effects of marine biofouling on gas sensor membrane materials

The use of underwater gaseous sensors has increased rapidly in the last 10 years. The majority of such sensors employ a thin membrane through which the gas diffuses. These sensors are potentiometric gas-sensing probes and essentially they are ion-selective electrodes. The deployment time of these membranes is curtailed by the formation of biofouling on the membrane leading to erroneous results. The physical properties of a variety of commonly used membranes were investigated using SEM and AFM. This showed that there were differences in topography between the PTFE membranes, such as pore sizes and surface roughness, which may be attributed to the manner in which they are manufactured. The pore size of the PTFE membranes varied greatly, ranging from circular pores with a diameter of 500 nm to elongated pores measuring 1 x 22 microm. The contact angle of each membrane showed that they were all hydrophobic. The amount of fouling on each was also observed and its affect on oxygen diffusion was monitored. Fouling slowed down the response of the instrument and caused reduced diffusion through the membranes. The amount of fouling varied between the membranes with the YSI membrane fouling least. Some of the membranes tested did foul less than others and there could be lifetime advantages of choosing a membrane with a smoother surface and a small pore size.

Effect of galvanically induced surface potentials on marine fouling

This paper reports the effect of a galvanically produced negative surface potential on the accumulation of marine biofouling. The potential was created by connecting pieces of copper or stainless steel to a layer of Indium/Tin Oxide semiconductor deposited on glass. It was shown that the negative potential significantly reduced the accumulation of biofouling. As the conductive layer is transparent, the technique was used to protect the windows of commercial optical instruments. This technology could provide an inexpensive way of extending deployment times for marine instruments.