Endotoxin or lipopolysaccharide (lps) is a major constituent of the gram-negative bacterial cell wall that causes a life-threatening disorder called septicemia resulting from the unregulated activation of the innate immune system. we have developed a simple colorimetric assay for the detection of lps in human blood/sera and water in which lps is captured from an analyte on a long-chain silane-functionalized glass substrate and tagged with gold nanoparticle probes surface-engineered with a narrow spectrum antibiotic drug, polymyxin b, that has high specificity for the lps molecules. the signal sensitivity after gnp binding is further amplified using a 30 second silver reduction step that produces colorimetric spots in the upper femto-molar range but still visible to the naked eye. the color intensity of the spots is also quantified using an led-smart phone-based optical acquisition system and a matlab image-processing code. the dynamic range of the assay is observed to lie between 100 fg/ml and 1 ng/ml and its lowest limit of detection is 10 fg/ml. the effects of various experimental parameters such as silane concentration, incubation time, temperature and humidity are optimized in detail to determine the best operating conditions to perform our assay. in addition, the effect of a chemical pre-concentration step is included in our study to see whether the net sensitivity of detection can be enhanced even beyond the clinical limit to allow pre-symptomatic (sub-clinical) stage monitoring and screening. finally, a calibration curve is generated from spiked endotoxin levels to determine their unknown concentration in clinical samples. the results are benchmarked to a commercial elisa kit and other patient data (microbiology, cbc, haematology reports etc.). overall, our approach is simple, sensitive, robust and cost-effective suggesting great utility for clinical usage.