Penicillin is a group of antibiotics classified specifically as β-lactam antibiotics due to the presence of a β-lactam ring in its molecular structure, and has been widely used since its discovery in 1928.1,2 The facile analysis of penicillin has thus been of importance in various fields of applications including medical science, medicine, pharmaceutical industry, food industry, and environmental monitoring.3-6 Several sensing strategies have been developed for the detection of penicillin; they can be categorized mainly into three general approaches. The first involves the use of pH-sensitive indicator dyes. This type of penicillin sensor is based on the enzymatic hydrolysis of penicillin to penicilloic acid, resulting in a pH change that is sensed optically by means of an acid-base indicator.3 The second approach utilizes pH glass electrodes. The principle in the operation of this penicillin sensor is that the pH glass electrode senses changes in the H + concentration resulting from the catalyzed hydrolysis of penicillin by penicillinase. 7 The final approach is based on the principle of the field effect in semiconductor structures. This type of penicillin sensor provides many advantages over the first two types, mostly due to its potential for miniaturization and integration and also due to its rapid response. However, the development of a sensitive penicillin sensor that allows the detection of small amounts of penicillin at low micromolar levels rapidly and inexpensively is critically needed in order to meet the practical needs in many fields of application such as pharmaceutical production, clinical analysis of antibiotic tablets, and food control. 5,8,9 In this context, we report an electrochemical penicillin sensor utilizing dendrimer-encapsulated Pt nanoparticles (Pt DENs) as electrocatalysts for the sensitive detection of penicillin. As illustrated in Scheme 1, the electrochemical penicillin sensor was prepared using a screen-printed carbon electrode (SPCE), on which both Pt DEN and penicillinase were immobilized. Upon the addition of penicillin, the immobilized penicillinase catalyzes the hydrolysis of penicillin to penicilloic acid, resulting in the production of H + ions which are electrochemically detected by means of the catalyzed electrochemical reduction of the generated H + ions on the Pt DEN-modified SPCE surface. Specifically, we synthesized Pt DENs containing an average of 147 atoms using amineterminated sixth-generation polyamidoamine dendrimers, which we denote as G6-NH 2 (Pt 147 ), and immobilized the Pt DENs with penicillinase enzymes onto SPCE surfaces. The resulting SPCE exhibited highly enhanced electrocatalytic activity for the electrochemical reduction of H + ions, leading to significantly increased cathodic current of H + . Especially, the Pt DEN-modified SPCE resulted in the electrochemical detection of penicillin as low as 0.1 μM, which allowed sensitive analysis of penicillin even in human blood serum.Figure 1(a) shows transmission electron microscopy (TEM) images of the as-synthe...